Chelsea wrote several equations and determined that only one of the equations has exactly one solution. Which of these equations has exactly one solution?
4 (x + 3) = 7 x minus 6
4 (x + 3) + 8 = 4 x + 20
4 (x + 3) + 3 x + 7 x + 12
4 (x + 3) + 6 = 4 x + 9