\[\frac{72a^{5}b^{4}}{25y^{8}}\ :\left( 24a^{7}b^{9} \right) =\]
\[= \frac{72a^{5}b^{4}}{25y^{8} \cdot 24a^{7}b^{9}} = \frac{3}{25y^{8}a^{2}b^{5}}\]