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 $2^{2}$.
$$2\sqrt{3}\sqrt{18}$$
Factor $18=3^{2}\times 2$. Rewrite the square root of the product $\sqrt{3^{2}\times 2}$ as the product of square roots $\sqrt{3^{2}}\sqrt{2}$. Take the square root of $3^{2}$.
$$2\sqrt{3}\times 3\sqrt{2}$$
Multiply $2$ and $3$ to get $6$.
$$6\sqrt{3}\sqrt{2}$$
To multiply $\sqrt{3}$ and $\sqrt{2}$, multiply the numbers under the square root.
