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

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

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