\[5a^{2}b = 8 \Longrightarrow \ \ a^{2}b^{3} = \frac{8}{5}.\]
\[- \frac{5}{3}a^{4}b^{6} = - \frac{5}{3}\left( a^{2}b^{3} \right)^{2} =\]
\[= \frac{- 5 \cdot 64}{3 \cdot 25} = - \frac{64}{15} = - 4\frac{4}{15}.\]