Answer
$-\dfrac{n^2}{9}$
Work Step by Step
Because the least common denominator is $18$, we create a common denominator of $18$ and obtain:
\begin{array}{l}\require{cancel}
\dfrac{9(-1n^2)+3(5n^2)-2(4n^2)}{18}
\\\\=
\dfrac{-9n^2+15n^2-8n^2}{18}
\\\\=
\dfrac{-2n^2}{18}
\\\\=
\dfrac{-\cancel{2}n^2}{\cancel{2}\cdot9}
\\\\=
-\dfrac{n^2}{9}
.\end{array}
