Consider $\left(\sqrt{6}+\sqrt{3}\right)\left(\sqrt{6}-\sqrt{3}\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
Consider $\left(\sqrt{6}+\sqrt{2}\right)\left(\sqrt{6}-\sqrt{2}\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
Consider $\left(\sqrt{6}-\sqrt{2}\right)\left(\sqrt{6}+\sqrt{2}\right)$. Multiplication can be transformed into difference of squares using the rule: $\left(a-b\right)\left(a+b\right)=a^{2}-b^{2}$.
To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $3$ and $4$ is $12$. Multiply $\frac{3\sqrt{2}\left(\sqrt{6}-\sqrt{3}\right)}{3}$ times $\frac{4}{4}$. Multiply $\frac{4\sqrt{3}\left(\sqrt{6}-\sqrt{2}\right)}{4}$ times $\frac{3}{3}$.
Since $\frac{4\times 3\sqrt{2}\left(\sqrt{6}-\sqrt{3}\right)}{12}$ and $\frac{3\times 4\sqrt{3}\left(\sqrt{6}-\sqrt{2}\right)}{12}$ have the same denominator, add them by adding their numerators.
Since $\frac{4\left(2\sqrt{3}-2\sqrt{6}+3\sqrt{2}\right)}{4}$ and $\frac{4\sqrt{2}\left(\sqrt{6}+\sqrt{2}\right)}{4}$ have the same denominator, subtract them by subtracting their numerators.