Applying Case 3 of Master Theorem to $T(n) = 9T(n/3) + n^3$
Your statement of the regularity condition has a typo: it suffices to show that there exists a constant $c &lt; 1$ such that for large enough $n$,
$$
af(n/b) \leq cf(n).
$$
Substituting $a,b,f$, we have to show that for some constant $c &lt; 1$,
$$
9(n/3)^3 \le c n^3.
$$
You take it from here.