7(3x-4) = 6x -4 +3x +12
Distributing:
7(3x) -7(4) = 6x -4 +3x +12
21x - 28 = 6x -4 +3x +12
Combining similar terms:
21x - 28 = (6x + 3x) + (-4 +12)
21x - 28 = 9x + 8
9x is adding on the right, then it will subtract on the left
28 is subtracting on the left, then it will add on the right
21x - 9x = 8 + 28
12x = 36
12 is multiplying on the left, then it will divide on the right
x = 36/12
x = 3
