Hello.

I don't know how to calculate given coefficients in a given series. For example

$\displaystyle (x^{3}+x^4+x^5+...)$

coefficient next to $\displaystyle x^{13}=?$

or a different one:

$\displaystyle (2+3x)^3 \sqrt{2+x}$

coefficient next to $\displaystyle x^5=?$

Could you explain to me how it is done?