First of all you need to make sure that each side has only letters and numbers, for this equation you first need to subtract 6 of each side, this way you can change the equation appearence without changing it's value, because if you do something in both sides of an equation, it's ok. After subtracting 6 of each side you can divide both sides for -4, this way you'll take only x in the left side and 12/-4 on the other, this way you will see that x = -3
6 - 4x = 18
6 - 4x - 6 = 18 - 6
-4x = 12
-4x/-4 = 12/-4
x = -3
