Answer
$x=\dfrac{s-2r}{3}$
Work Step by Step
Subtract $2r$ from each side:
$\begin{align*}s-2r&=2r+3x-2r\\
s-2r&=3x
\end{align*}$
Divide both sides of the equation by $3$:
$\begin{align*}
\frac{s-2r}{3}&=\frac{3x}{3}\\
\frac{s-2r}{3}&=x\\
\end{align*}$
