If f(x) = 2 /3 x + 9, find f−1(x).
A) f square root −1(x) = 3x − 9
B) f square root −1(x) = 2 /3 x + 9
C) f square root −1(x) = 3 /2 x + 27/ 2
D) f square root −1(x) = 3/ 2 x − 27/ 2