Factor $50=5^{2}\times 2$. Rewrite the square root of the product $\sqrt{5^{2}\times 2}$ as the product of square roots $\sqrt{5^{2}}\sqrt{2}$. Take the square root of $5^{2}$.
$$5\sqrt{2}-\sqrt{2}$$
Combine $5\sqrt{2}$ and $-\sqrt{2}$ to get $4\sqrt{2}$.
