This exercise is asking you to find the value of x or solve for x, to do so you have to isolate or leave x alone by doing different mathematical operations according to the given equation.
The given equation is -x=3-4x+15, which is a three step equation meaning that in order to solve for x you have to perform three different mathematical operations, in this case combining of like terms, addition, and division.
-x=3-4x+15 Combine like terms
-x=-4x+18 Add 4x in both sides
3x=18 Divide by 3 in both sides to isolate x
x=6
The value of x in the equation is 6 or letter C from the given choices.