Intermediate Algebra, Chapter 2, Review Exercises, Section Review Exercises, Problem 10

Evaluate the equation $\displaystyle M = -\frac{1}{4}(x + 3y)$ for $x$

$
\begin{equation}
\begin{aligned}
M(-4) &= -\frac{1}{4} (x + 3y) (-4)
&& \text{Multiply each side by } -4\\
\\
-4M &= x + 3y
&& \text{Evaluate}\\
\\
-4M - 3y &= x + 3y - 3y
&& \text{Subtract each side by } 3y\\
\\
-4M - 3y &= x
&& \text{Solve for } x
\end{aligned}
\end{equation}
$