Equations can contain more than one variable. For example, the following equation has two variables, \color{blue}x and {\color{red}y}{:}

{\color{blue}x} - 7{\color{red}y} = 1

To solve the equation for the variable {\color{blue}{x}}, we need to isolate \color{blue}x on one side of the equation:

In this case, we apply the addition principle and add 7{\color{red}y} to both sides:

\begin{align*} \require{cancel} {\color{blue}x} - 7{\color{red}y} &= 1 \\[5pt] {\color{blue}x} - 7{\color{red}y}+7{\color{red}y} &= 1+7{\color{red}y} \\[5pt] {\color{blue}x}+0 &= 1+7{\color{red}y} \\[5pt] {\color{blue}x} &= 1+7{\color{red}y} \end{align*}

And that's it! We have successfully isolated the variable {\color{blue}{x}}.

Let's now look at an example where we must apply the multiplication principle.

FLAG

Solve the equation 4x = y for the variable x.

EXPLANATION

We need to isolate the variable x. To do this, we treat y as though it were a constant and apply the addition and multiplication principles as usual.

In this case, we simply divide both sides of the equation by 4\mathbin{:}

\eqalign{ \require{cancel} 4x &= y \\[5pt] \dfrac {4x} {4} &= \dfrac y 4 \\[5pt] \dfrac {{\cancel{4}}x} {\cancel{4}} &= \dfrac y 4 \\[5pt] x &= \dfrac y4 }

FLAG

If $h-\dfrac{3}{2}k=2$ then $h=$

a
$2+\dfrac{3}{2}k$
b
$2+3k$
c
$2+2k$
d
$2-\dfrac{3}{2}k$
e
$\dfrac{3}{2}k$

If $x+k=5,$ then $x=$

a
$5+k$
b
$k-5$
c
$5-k$
d
$\dfrac{5}{k}$
e
$-5k$

If $\dfrac{x}{5}=y,$ then $x=$

a
$y-5$
b
$5y$
c
$y+5$
d
$\dfrac 5y$
e
$\dfrac y5$

Given that \dfrac{x}{3} + 2y = 1, express x in terms of y.

EXPLANATION

We need to isolate the variable x. To do this, we treat y as though it were a constant and apply the addition and multiplication principles as usual.

First, we subtract 2y from both sides of the equation: \begin{align*} \dfrac{x}{3} + 2y & = 1\\[5pt] \dfrac{x}{3} + 2y - 2y & = 1-2y\\[5pt] \dfrac{x}{3}+0 & = 1-2y\\[5pt] \dfrac{x}{3} & = 1-2y \end{align*}

Then, we multiply both sides of the equation by 3\mathbin{:} \begin{align*} \require{cancel} \dfrac{x}{3} &= 1-2y\\[5pt] 3 \cdot \dfrac{x}{3} &= 3 \cdot (1-2y) \\[5pt] \cancel{3}\cdot \dfrac{x}{\cancel{3}}&= 3 \cdot (1-2y) \\[5pt] x &= 3 \cdot (1-2y) \\[5pt] x &= 3 \cdot 1 - 3 \cdot 2y \\[5pt] x &= 3-6y \end{align*}

FLAG

If $3x-4y=5y$ then $x=$

a
$3y$
b
$3x$
c
$\dfrac{y}{3}$
d
$-3y$
e
$-\dfrac{y}{3}$

If $\dfrac{s}{4}+\dfrac{t}{2}=1$ then $s=$

a
$4 + 2t$
b
$4 + \dfrac{t}{2} $
c
$2 - \dfrac{t}{2} $
d
$1 - \dfrac{t}{2} $
e
$4 - 2t$

If $3a + b = 12a,$ then $a = $

a
$\dfrac b {15}$
b
$15b$
c
$\dfrac b 9$
d
$9b$
e
$\dfrac 9 b$

Solve the following equation for the variable x\mathbin{:}

\dfrac{3x-2y}{3}=\dfrac{1+2y}{4}

EXPLANATION

We need to isolate the variable x. First, we cross-multiply:

\begin{align*} \dfrac{3x-2y}{3} &=\dfrac{1+2y}{4} \\[5pt] 4\cdot (3x-2y) &= 3\cdot (1+2y) \\[5pt] 12x - 8y &= 3+6y \\[5pt] \end{align*}

Next, we add 8y to both sides of the equation:

\begin{align*} 12x - 8y &= 3+6y \\[5pt] 12x - 8y + 8y &= 3+6y + 8y \\[5pt] 12x &= 3+14y \end{align*}

Finally, we divide both sides of the equation by 12\mathbin{:}

\begin{align*} \require{cancel} 12x &= 3+14y \\[5pt] \dfrac{12x}{12} &= \dfrac{3+14y}{12} \\[5pt] \dfrac{\cancel{12}x}{\cancel{12}} &= \dfrac{3+14y}{12} \\[5pt] x &= \dfrac{3+14y}{12} \end{align*}

FLAG

If $4m - 6n = 3n - 7$, then $n=$

a
$\dfrac{m + 7}{9}$
b
$\dfrac{4m + 7}{9}$
c
$\dfrac{4m - 7}{9}$
d
$\dfrac{2m + 7}{9}$
e
$\dfrac{2m - 7}{9}$

If $\dfrac{4x-3y}{2}=\dfrac{1-y}{3},$ then $x=$

a
$- \dfrac{2x+7}{12y}$
b
$\dfrac{2 +7y}{12}$
c
$\dfrac{7}{12}$
d
$\dfrac{7 -2y}{4}$
e
$\dfrac{7y}{3}$

If $\dfrac{f - 2}{5} = \dfrac{5g - 2f}{6},$ then $f=$

a
$ \dfrac{g + 12}{5}$
b
$ \dfrac{21g + 12}{12}$
c
$ \dfrac{23g - 12}{10}$
d
$ \dfrac{25g + 12}{16}$
e
$ \dfrac{24g + 2}{6}$
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