Let's suppose that we want to solve an equation like

\dfrac x 6 = 4.

For this, we can use the multiplication principle. The multiplication principle tells us that multiplying (or dividing) both sides of an equation by the same number does not change its solution.

In this case, the variable x is being divided by 6. To isolate x , we can perform the opposite operation, which is multiplying by 6 . So, we multiply both sides of the equation by 6.

\eqalign{ \dfrac x 6 &= 4 \\[5pt] {\color{blue}{6}} \cdot \dfrac x 6 &= {\color{blue}{6}}\cdot 4\\[5pt] \dfrac{6x}{6} &=24\\[5pt] }

Now, we can cancel a common factor of 6 from both the numerator and denominator.

\eqalign{ \dfrac{6x}{6} &=24 \\ \dfrac{ \cancel{6} x}{\cancel{6} } &=24 \\ x &=24 }

Thus, our solution is x=24. Let's check that this solution is correct by plugging it back into the original equation:

\eqalign{ \dfrac x 6 &=4 \\[5pt] \dfrac{24}{6} &=4 \\[5pt] 4 &= 4 \; {\color{green}{\checkmark}} }

FLAG

Find the value of q that satisfies the equation \dfrac{q}{4}=-9.

EXPLANATION

In the equation, q is being divided by 4. To isolate q , we can perform the opposite operation, which is multiplying by 4 . So, we multiply both sides of the equation by 4.

\require{cancel} \eqalign{ \dfrac{q}{4} &=-9\\[5pt] 4\cdot \dfrac{q}{4} &=4\cdot \left(-9\right) \\[5pt] \dfrac{4q}{4} &= -36 }

Now, we can cancel a common factor of 4 from both the numerator and denominator.

\require{cancel} \eqalign{ \dfrac{4q}{4} &= -36 \\[5pt] \dfrac{\cancel{4}q}{\cancel{4}} &= -36 \\ q &=-36 }

FLAG

If $\dfrac m 8= 11$, then $m = $

a
$\dfrac {11} 8$
b
$-3$
c
$3$
d
$19$
e
$88$

If $\dfrac{z}{6}=-6,$ then $z=$

a
$z=-36$
b
$z=-1$
c
$z=1$
d
$z=\dfrac{1}{2}$
e
$z=-12$

If \dfrac{1}{7}=\dfrac{x}{2}, then x=

EXPLANATION

First, let's swap the left and right-hand sides so that the variable is on the left-hand side.

\dfrac{x}{2}=\dfrac{1}{7}

In this equation, x is being divided by 2. To isolate x , we can perform the opposite operation, which is multiplying by 2. So, we multiply both sides of the equation by 2 and simplify.

\begin{align*} \require{cancel} \dfrac{x}{2} &=\dfrac{1}{7} \\[5pt] 2 \cdot \dfrac{x}{2} &=2 \cdot\left(\dfrac{1}{7}\right) \\[5pt] \dfrac{2x}{2} &= \dfrac{2}{7} \\[5pt] \dfrac{\cancel{2}x}{\cancel{2}} &= \dfrac{2}{7} \\[5pt] x &= \dfrac{2}{7} \end{align*}

FLAG

If $\dfrac{1}{4}=\dfrac{g}{12},$ then $g=$

a
$6$
b
$3$
c
$-8$
d
$-3$
e
$8$

If $-\dfrac{1}{3}=\dfrac{m}{6},$ then $m=$

a
$-\dfrac{2}{3}$
b
$2$
c
$-\dfrac{1}{18}$
d
$-2$
e
$\dfrac{1}{18}$

In order to remove a negative sign from our variable, we can multiply both sides by -1 . This way, the two negatives will cancel out to form a positive.

For example, suppose that we have the equation

-x = 3

We can isolate x by multiplying both sides of the equation by -1, as follows:

\begin{align*} -x &= 3 \\[5pt] (-1) \cdot (-x) &= (-1) \cdot 3 \\[5pt] x &= -3 \end{align*}

Therefore, the solution to the equation is x=-3. Let's check to see that this works by substituting our solution back into the original equation.

\begin{align*} -x &= 3\\[5pt] -(-3) &=3\\[5pt] 3 &=3\quad{\color{green}{\checkmark}} \end{align*}

FLAG

Find the value of k that satisfies the equation -\dfrac{k}{2}=\dfrac{1}{3}.

EXPLANATION

First, let's remove the negative sign from the variable. To do this, we multiply both sides of the equation by -1.

\begin{align*} -\dfrac{k}{2} &= \dfrac{1}{3} \\[5pt] (-1) \cdot \left( -\dfrac{k}{2} \right) &= (-1) \cdot \left( \dfrac{1}{3} \right) \\[5pt] \dfrac{k}{2} &= -\dfrac{1}{3} \end{align*}

In the equation above, k is being divided by 2. To isolate k , we can perform the opposite operation, which is multiplying by 2 . So, we multiply both sides of the equation by 2 and simplify.

\require{cancel} \eqalign{ \dfrac{k}{2} &=-\dfrac{1}{3} \\[5pt] 2 \cdot \dfrac{k}{2} &=2 \cdot \left( -\dfrac{1}{3} \right) \\[5pt] \dfrac{2k}{2} &= -\dfrac{2}{3} \\[5pt] \dfrac{\cancel{2}k}{\cancel{2}} &= -\dfrac{2}{3} \\[5pt] k &= -\dfrac{2}{3} }

FLAG

If $-\dfrac{c}{5}=3,$ then $c=$

a
$-15$
b
$-\dfrac{5}{3}$
c
$15$
d
$-\dfrac{3}{5}$
e
$\dfrac{3}{5}$

If $-\dfrac{p}{3}=\dfrac{1}{6},$ then $p=$

a
$-2$
b
$\dfrac{1}{2}$
c
$-\dfrac{1}{2}$
d
$\dfrac{1}{18}$
e
$-\dfrac{1}{18}$

To solve an equation like 7x = 21 , we again need to look at what's happening to the variable and ask ourselves what the opposite operation is.

In this case, the variable x is being multiplied by 7. To isolate x , we can perform the opposite operation, which is dividing by 7 . So, we divide both sides of the equation by 7.

\eqalign{ 7x &= 21 \\[5pt] \dfrac{7x}{{\color{blue}7}} &=\dfrac{21}{{\color{blue}7}} }

Now, we can cancel a common factor of 7 from both the numerator and denominator.

\eqalign{ \dfrac{7x}{{\color{blue}7}} &=\dfrac{21}{{\color{blue}7}}\\[5pt] \dfrac{\cancel{7}x}{\cancel{\color{blue}7}} &=3 \\[5pt] x &=3 }

Let's check that our answer is correct by plugging it back into the original equation:

\eqalign{ 7x &=21 \\[5pt] 7(3) &=21 \\[5pt] 21 &= 21 \; {\color{green}{\checkmark}} }

Note: If there is a negative coefficient on a variable, we can get rid of it by dividing, like we do with positive coefficients. For example:

\begin{align*} \require{cancel} -2y &= 10 \\[5pt] \dfrac{-2y}{-2} &= \dfrac{10}{-2} \\[5pt] \dfrac{\cancel{-2}y}{\cancel{-2}} &= -5 \\[5pt] y &= -5 \end{align*}

FLAG

Solve the equation 24=-3z.

EXPLANATION

First, let's write the equation in a way that's a little more familiar to us. We can do this by swapping the left and right-hand sides of the equation.

\begin{align*} 24 &=-3z \\ -3z &= 24 \end{align*}

In this case, the variable z is being multiplied by -3. To isolate z , we can perform the opposite operation, which is dividing by -3 . So, we divide both sides of the equation by -3 and simplify.

\require{cancel} \eqalign{ -3z &=24 \\[5pt] \dfrac {-3z} {-3} &= \dfrac {24} {-3} \\[5pt] \dfrac {\cancel{-3}z} {\cancel{-3}} &= -8 \\[5pt] z &=-8 }

FLAG

If $6y = 14$, then $y = $

a
$\dfrac 3 7$
b
$20$
c
$\dfrac 7 3$
d
$8$
e
$7$

If $-4x = -36$, then $x=$

a
$-9$
b
$-4$
c
$9$
d
$4$
e
$-40$
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