To multiply $\sqrt{21}$ and $\sqrt{14}$, multiply the numbers under the square root.
$$\sqrt{294}$$
Factor $294=7^{2}\times 6$. Rewrite the square root of the product $\sqrt{7^{2}\times 6}$ as the product of square roots $\sqrt{7^{2}}\sqrt{6}$. Take the square root of $7^{2}$.
