Let y = f(x)
Let cr = cube root for short
y = cr{3x} + 2
Switch x and y.
x = cr{3y} + 2
Solve for y.
x - 2 = cr{3y}
Raise both sides to the 3rd power.
(x - 2)^3 = [cr{3y}]^3
(x - 2)^3 = 3y
[(x - 2)^3]/3 = y
Replace y with the inverse function notation
f^(-1) x.
f^(-1) x = [(x - 2)^3]/3
Choice b is the answer.