How can you find the inverse $\displaystyle f^{-1}(x)$ of $\displaystyle f(x)=x^2+3x$ by restricted the domain?

I looked at this and expected to end up with a square root of something, so that you could restrict the domain to only include positive or negative values, but then I tried to solve $\displaystyle f(f^{-1}(x))=x$, and couldn't. Can anyone help?