Answer: x = 27
Multiple both sides by the equation of 3:
4x - 12 = 15 + 3x
Move the variable to the left and change the sign: 4x - 3x - 12 = 15
Move the constant to the right hand and change the sign: 4x - 3x = 15 + 12
Collect like terms: x = 15 + 12
Get the answer: x = 27