jmc

Simplify $\sqrt{20}$ as $\sqrt{2^2}\cdot \sqrt{5}=2\sqrt{5}$. Also simplify $\sqrt{45}$ as $\sqrt{3^2}\cdot \sqrt{5}=3\sqrt{5}$. The desired expression is $\sqrt{5}-2\sqrt{5}+3\sqrt{5}=\boxed{2\sqrt{5}}$.