$5 \sqrt{3}$

#### Explanation:

We have:

$\sqrt{12} + 3 \sqrt{3}$

We can rewrite the $\sqrt{12}$ to put it into terms of $\sqrt{3}$:

$\sqrt{4 \times 3} + 3 \sqrt{3}$

$\sqrt{4} \times \sqrt{3} + 3 \sqrt{3}$

$2 \sqrt{3} + 3 \sqrt{3}$

and now combine:

$5 \sqrt{3}$