Find $\displaystyle (f^{-1})(a)$ for the function $\displaystyle f$ and real number $\displaystyle a$.

$\displaystyle f(x)=x^3+2x-1$, $\displaystyle a=2$

$\displaystyle f(x)=\frac{1}{27}(x^5+2x^3)$, $\displaystyle a=-11$

I'm having trouble with these. I've got a test on monday and I'm freaking out. The biggest problem that I'm having is finding the number that corresponds to a. I can't sub for $\displaystyle f(x)$ because the function can't be solved easily in most cases, and I can't determine the inverse function by switching variables and solving for y. What do I do?