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 $2^{2}$.
$$-240\times 2\sqrt{5}\sqrt{6}$$
Multiply $-240$ and $2$ to get $-480$.
$$-480\sqrt{5}\sqrt{6}$$
To multiply $\sqrt{5}$ and $\sqrt{6}$, multiply the numbers under the square root.
