I have such $$[x^8](1-5x^4)^{-2}(1+3x)^{-6}$$ I simplify as $$[x^8]\sum_{n \geq 0} \binom{n+2-1}{2-1}(5x^4)^n \cdot \sum_{n \geq 0} \binom{n+6-1}{6-1}(-3x)^n $$
$$[x^8]\sum_{n \geq 0} \binom{n+1}{1}5^n(x^{4n}) \cdot \sum_{n \geq 0} \binom{n+5}{5}(-3^n)x^n $$
I don't know what to do with the multiplication in there and the further steps
Thanks in advance.