first of all it is best to get rid of the brackets. You do this by times each component in the bracet by the number on the outside in this case it is 2.
7x + 8 = 12x - 4
then you want to get all the x's on one side and the numbers on the other side. to do this, i added 4 to both sides and -7x from both sides. I did this so there was no negative numbers on eitherside to make it a little easier.
12 = 5x
the next step is to get x on its on. you do this by deviding both sides by 5, there fore;
x = 12/5