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