AMC 12 2000 Problem 15

Let f be a function for which f(x / 3)=x^{2}+x+1. Find the sum of all values of z for which f(3 z)=7.

Answer Choices
A. -1 / 3
B. -1 / 9
C. 0
D. 5 / 9
E. 5 / 3