#### Answer

$m=-5$

#### Work Step by Step

We start with the given equation: $8+3m=-7$
We start by subtracting $8$ from each side of the equation: $8-8+3m=-7-8$
Next, we combine like terms: $3m=-15$
Finally, we divide by $3$ on each side of the equation: $m=\frac{-15}{3}=-5$