If f(x) = 3x + 6, which of the following is the inverse of f(x)?

A. f⁻¹(x) = 6-3x
B. f⁻¹(x) = (x-6) /3
C. f⁻¹(x) = (x-3) /6
D. f⁻¹(x) = 3x - 6