Answer
$x=-2$
Work Step by Step
The function is $m(x)=4x+15$.
Substitute $7$ for $m(x)$ to obtain
$\implies 7=4x+15$
Subtract $15 $ from both sides of the equation.
$7-15=4x+15-15$
$\implies -8=4x$
Divide both sides of the equation by $4$ to get
$\frac{-8}{4}=\frac{4x}{4}$
$\implies x=-2$
When $x=-2$, $m(x)=7$
