Answer
$\color{blue}{\dfrac{2x}{(x-z)(x+z)}}$
Work Step by Step
The expressions are not similar since they have different denominators.
Make the expressions similar using their LCD $(x+z)(x-z)$ to obtain:
$=\dfrac{1\color{red}{(x-z)}}{(x+z)\color{red}{(x-z)}} + \dfrac{1\color{red}{(x+z)}}{(x-z)\color{red}{(x+z)}}
\\=\dfrac{x-z}{(x+z)(x-z)}+\dfrac{x+z}{(x-z)(x+z)}$
Add the numerators and copy the denominators to obtain:
$=\dfrac{(x-z)+(x+z)}{(x-z)(x+z)}
\\=\dfrac{(x+x)+(z-z)}{(x-z)(x+z)}
\\=\color{blue}{\dfrac{2x}{(x-z)(x+z)}}$
