Factor $45=3^{2}\times 5$. Rewrite the square root of the product $\sqrt{3^{2}\times 5}$ as the product of square roots $\sqrt{3^{2}}\sqrt{5}$. Take the square root of $3^{2}$.
$$-3\sqrt{5}+\sqrt{20}$$
Factor $20=2^{2}\times 5$. Rewrite the square root of the product $\sqrt{2^{2}\times 5}$ as the product of square roots $\sqrt{2^{2}}\sqrt{5}$. Take the square root of $2^{2}$.
$$-3\sqrt{5}+2\sqrt{5}$$
Combine $-3\sqrt{5}$ and $2\sqrt{5}$ to get $-\sqrt{5}$.
