How do you solve -15x-40=9-8x?

Explanation:
Given
-15x-40=9-8x
Add 8x+40 (=40 + 8x) to both sides in order to isolate the variable term on the left and the constant term on the right:
-15x-40=9-8x
$$\begin{gathered} +\underset{―}{8x+40}=\underset{―}{40+8x} \\ -7x=49 \end{gathered}$$
Divide both sides by (-7)
x=7

Explanation:
First, add 15x and subtract 9 from each side of the equation to isolate the x term while keeping the equation balanced:
-15x-40+15x-9=9-8x+15x-9
-15x+15x-40-9=9-9-8x+15x
0-49=0+7x
-49=7x
Now, divide each side of the equation by 7 to solve for x while keeping the equation balanced:
${\displaystyle -\frac{49}{7}=\frac{7x}{7}}$
$\text{Undefined control sequence cancel}$
-7=x
x=-7