To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $8$ and $2$ is $8$. Multiply $\frac{3\sqrt{2}+\sqrt{6}}{2}$ times $\frac{4}{4}$.
Since $\frac{\sqrt{2}\left(1+\sqrt{3}\right)}{8}$ and $\frac{4\left(3\sqrt{2}+\sqrt{6}\right)}{8}$ have the same denominator, subtract them by subtracting their numerators.