Answer
$\dfrac{n^{12}}{m^{3}}$
Work Step by Step
Using the laws of exponents, the given expression, $
\left( \dfrac{m^{-1}}{n^{-4}} \right)^{3}
$, is equivalent to
\begin{array}{l}\require{cancel}
\left( \dfrac{n^{4}}{m^{1}} \right)^{3}
\\\\=
\dfrac{n^{4(3)}}{m^{1(3)}}
\\\\=
\dfrac{n^{12}}{m^{3}}
.\end{array}