Consider $\left(9+2\sqrt{14}\right)\left(9-2\sqrt{14}\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
Factor $84=2^{2}\times 21$. Rewrite the square root of the product $\sqrt{2^{2}\times 21}$ as the product of square roots $\sqrt{2^{2}}\sqrt{21}$. Take the square root of $2^{2}$.
