Add 4 to both sides
x = 4 + 3 x + 12
combine like terms (4 and 12)
x = 3 x + 16
subtract 3 x from both sides
- 4 x = 16
divide both sides by - 4
x = -4

Add 4 to both sides
x = 4 + 3 x + 12
combine like terms (4 and 12)
x = 3 x + 16
subtract 3 x from both sides
- 2 x = 16
divide both sides by - 2
x = -8