\[к\ знаменателю\ 54m^{3}n^{6};\]
\[\frac{4}{9m^{2}n} = \frac{4 \cdot 6mn^{5}}{9m^{2}n \cdot 6mn^{5}} =\]
\[= \frac{24mn^{5}}{54m^{3}n^{6}}\]