Answer
$\color{blue}{\dfrac{a-b}{a^2}}$
Work Step by Step
The expressions are not similar since they have different denominators.
Make the expressions similar using their LCD which is $a^2$ to obtain:
$=\dfrac{1\color{red}{(a)}}{a\color{red}{(a)}}-\dfrac{b}{a^2}
\\=\dfrac{a}{a^2}-\dfrac{b}{a^2}
\\=\color{blue}{\dfrac{a-b}{a^2}}$
