13. $m(x)=4x + 15; m(x)=7$

Explanation:

Step1: Substitute $m(x)$ value

Since $m(x)=4x + 15$ and $m(x)=7$, we set up the equation $4x+15 = 7$.

Step2: Isolate the term with $x$

Subtract 15 from both sides: $4x=7 - 15$. $4x=-8$.

Step3: Solve for $x$

Divide both sides by 4: $x=\frac{-8}{4}$. $x=-2$.

Answer:

$x = - 2$