You can work this out by rearranging the equation and isolating the x.
4x + 12 = 36
4x = 36 - 12 (I moved the +12 to the other side, which means it becomes -12)
4x = 24 (36 - 12 = 24)
x = 24 / 4 (You need to isolate the x, and because it was being multiplied by 4, I divided both sides by 4 to leave me with just x)
x = 6 (24 divided by 4 is 6).
