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 $f(3z) = 7$.

$\textbf {(A)}\ -1/3 \textbf {(B)}\ -1/9 \textbf {(C)}\ 0 \textbf {(D)}\ 5/9 \textbf {(E)}\ 5/3$