First find the value of "x", by isolating/getting the variable "x" by itself in the equation:
x - 12 = -15 Add 12 on both sides to get "x" by itself in the equation
x - 12 + 12 = -15 + 12
x = -3
Now that you found the value of x, you can use it to find the value of 4 - x
4 - x Substitute/Plug in -3 for "x"
4 - (-3) [two negative signs cancel each other out and become positive]
4 + 3
7 The value of 4 - x is 7