Answer
$\dfrac{2}{7n}-\dfrac{3}{2m}+\dfrac{9}{7mn}$
Work Step by Step
Dividing the denominator to each of the terms of the numerator, the given expression, $
\dfrac{4m^2n^2-21mn^3+18mn^2}{14m^2n^3}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{4m^2n^2}{14m^2n^3}-\dfrac{21mn^3}{14m^2n^3}+\dfrac{18mn^2}{14m^2n^3}
\\\\=
\dfrac{2}{7n}-\dfrac{3}{2m}+\dfrac{9}{7mn}
.\end{array}