Simplify $\sqrt5-\sqrt{20}+\sqrt{45}$.
Simplify $\sqrt{20}$ as $\sqrt{2^2}\cdot\sqrt5 = 2\sqrt5$. Also simplify $\sqrt{45}$ as $\sqrt{3^2}\cdot\sqrt5 = 3\sqrt5$. The desired expression is $\sqrt5-2\sqrt5+3\sqrt5 = \boxed{2\sqrt5}$.