\[3\frac{1}{2}m^{- 8}n^{- 7}\ :\left( - \frac{7}{8}m^{- 5}n^{- 7} \right) =\]
\[= - \frac{7}{2} \cdot \frac{8}{7}m^{- 3}n^{0} = - 4m^{- 3}\]