Since you are solving for x, you need to isolate x on one side of the equation. Start by subtracting C from both sides.
0 = x + a / b - C
Subtract x from both sides to have it on the left side of the equation.
-x = a / b - C
Make x a positive number by dividing both sides by negative one.
x = - a / b + C
A better way of writing this answer would be: [tex]x=-\frac{a}{b} + C[/tex].
