Simplifying
x + -6 = 9

Reorder the terms:
-6 + x = 9

Solving
-6 + x = 9

Solving for variable 'x'.

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

Add '6' to each side of the equation.
-6 + 6 + x = 9 + 6

Combine like terms: -6 + 6 = 0
0 + x = 9 + 6
x = 9 + 6

Combine like terms: 9 + 6 = 15
x = 15

Simplifying
x = 15