Factor $27=3^{2}\times 3$. Rewrite the square root of the product $\sqrt{3^{2}\times 3}$ as the product of square roots $\sqrt{3^{2}}\sqrt{3}$. Take the square root of $3^{2}$.
$$3\sqrt{3}+\sqrt{3}$$
Combine $3\sqrt{3}$ and $\sqrt{3}$ to get $4\sqrt{3}$.
