The expression is given as;
15=9x -120
To solve this you take 120 to the left side of the equation
15+120 = 9x
135 = 9x -------- here you see the multiplication property of Equality
You find the value of x that can be substituted in the position of x multiplied by 9 to get 135
The multiplication property states that : if x = y then x*z = y*z
Answer
C