The formula is:
[tex]\dfrac{(3m^{-2}n)^{-3}}{6mn^{-2}}=\dfrac{1}{3m^{-2}n}\cdot\dfrac{1}{6mn^{-2}}\\
=\dfrac{m^2}{3n}\cdot\dfrac{n^2}{6m}\\
=\dfrac{n^2}{3n}\cdot\dfrac{m^2}{6m}\\
=\dfrac{n}{3}\cdot\dfrac{m}{6}\\
=\dfrac{nm}{18}.[/tex]
Do you have any question ?