Factor $8=2^{2}\times 2$. Rewrite the square root of the product $\sqrt{2^{2}\times 2}$ as the product of square roots $\sqrt{2^{2}}\sqrt{2}$. Take the square root of $2^{2}$.
$$\sqrt{2}+2\sqrt{2}$$
Combine $\sqrt{2}$ and $2\sqrt{2}$ to get $3\sqrt{2}$.
