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 $3^{2}$.
$$\frac{3\sqrt{2}}{\sqrt{3}\sqrt{2}}$$
To multiply $\sqrt{3}$ and $\sqrt{2}$, multiply the numbers under the square root.
$$\frac{3\sqrt{2}}{\sqrt{6}}$$
Rationalize the denominator of $\frac{3\sqrt{2}}{\sqrt{6}}$ by multiplying numerator and denominator by $\sqrt{6}$.
