Given:

$\displaystyle f(x)=\sqrt{x}$ and $\displaystyle g(x)=x^2-1$

How can I evaluate something like:

$\displaystyle f(g(2))$ or $\displaystyle g(f(0))$

I seem to get confused whenever they throw a number in the composite. Like I know how to evaluate $\displaystyle f(g(x))$ but I'm not sure what to do when they place a number in it. :/