What is the coefficient of x^5 in the binomial expression (x - 2x^-2)^20 when multiplied by (1+3x^3)

So far I have:

(20 over k)(-1)^k (2)^k x^(20-3k)

so then 20-3k = 5 such that k = 5

So then (20 over 5 )(-1)^5 (2)^5

But then I don't know how to handle multiplying it by ( 1 + 3x^3)