Answer
$\color{blue}{\dfrac{7-4a^2b}{18a^3b^2}}$
Work Step by Step
The expressions are not similar since they have different denominators.
Make the expressions similar using their LCD which is $18a^3b^2$ to obtain:
$=\dfrac{7}{18a^3b^2} - \dfrac{2\color{red}{(2a^2b)}}{9ab\color{red}{(2a^2b)}}
\\=\dfrac{7}{18a^3b^2}-\dfrac{4a^2b}{18a^3b^2}
\\=\color{blue}{\dfrac{7-4a^2b}{18a^3b^2}}$
