Power Series Coefficients Find the sum of the coefficients of $x^{20}$ and $x^{21}$ in the power series expansion of $\frac 1{(1-x^3)^4}$.

I don't know a lot on power series at the moment, and I was wondering how do I find the coefficients?
Thanks

HINT:
Let $y=x^3$ and expand $f(y)=(1-y)^{-4}$ in a Taylor series around $y=0$.  
This series will have terms for $x^{3n}$ only, which implies that the coefficient for $x^{20}$ is zero.
We note that the $n$'th coefficient is $(n+3)!/3!n!$, from which we see that the coefficient for $x^{21}$ is at $n=7$ or $\binom{10}{3}$.
