\[2\frac{1}{4}a^{5}b \cdot \left( \frac{2}{3}ab^{3} \right)^{3} =\]
\[= \frac{9}{4}a^{5}b \cdot \frac{8}{27}a^{3}b^{9} = \frac{2}{3}a^{8}b^{10}\]