Multiply $\frac{3\sqrt{2}+\sqrt{3}}{3}$ times $\frac{3\sqrt{2}-\sqrt{3}}{3}$ by multiplying numerator times numerator and denominator times denominator.
Consider $\left(3\sqrt{2}+\sqrt{3}\right)\left(3\sqrt{2}-\sqrt{3}\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.