Factor $12=2^{2}\times 3$. Rewrite the square root of the product $\sqrt{2^{2}\times 3}$ as the product of square roots $\sqrt{2^{2}}\sqrt{3}$. Take the square root of $2^{2}$.
$$\frac{3\times 2\sqrt{3}}{\sqrt{3}}$$
Multiply $3$ and $2$ to get $6$.
$$\frac{6\sqrt{3}}{\sqrt{3}}$$
Rationalize the denominator of $\frac{6\sqrt{3}}{\sqrt{3}}$ by multiplying numerator and denominator by $\sqrt{3}$.
