Simplifying
15 + 5x = 0

Solving
15 + 5x = 0

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-15' to each side of the equation.
15 + -15 + 5x = 0 + -15

Combine like terms: 15 + -15 = 0
0 + 5x = 0 + -15
5x = 0 + -15

Combine like terms: 0 + -15 = -15
5x = -15

Divide each side by '5'.
x = -3

Simplifying
x = -3