I've become a bit confused on a question that is:

let $\displaystyle f(x)=x^2-1$ and $\displaystyle g(x)=2x+1$

Calculate $\displaystyle fg(2x)$

I'm not too sure. Is it:

$\displaystyle f(2(2x)+1)$ or $\displaystyle f(2(2x+1))$?

Thanks very much!