Hello! I'm stuck on these questions... Can someone help?

First: how would you write f(x) = x / (1 + x^{3}) as a series?

I mean like

∞

Σ = .....

k=0

I know that 1/(1-x) =

∞

Σ x^{k}

k=0

But how do i convert this into f(x)?

Second: how would you find the radius of convergence of the following binomial series:

∞

Σ (3k k) x^{2k+1}

k=0

with (3k k) is like (n k) = n! / k!(n-k)!

I know how to do it when there's x^{k }in the sommation but with x^{2k+1}?

Thanks for the help, I've been spending hours on these two questions!