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