Answer
$x=\dfrac{12.5+b}{a}$
Work Step by Step
$a$ is a nonzero constant so both sides of the equation can be divided by $a$.
$$\begin{align*}
ax-b+b&=12.5+b &\text{Add $b$ to both sides,}\\
ax&=12.5+b &\text{Simplify.}\\
\frac{ax}{a}&=\frac{12.5+b}{a} &\text{Divide $a$ to both sides.}\\
x&=\frac{12.5+b}{a} &\text{Simplify.}
\end{align*}$$
