\[\left( \frac{5a^{- 3}}{b^{- 2}} \right)^{- 3} \cdot \left( 25a^{- 8}b^{5} \right)^{2} =\]
\[= \frac{5^{- 3}a^{9}}{b^{6}} \cdot 25^{2}a^{- 16}b^{10} =\]
\[= \frac{5^{- 3} \cdot a^{9} \cdot \left( 5^{2} \right)^{2}b^{10}}{b^{6} \cdot a^{16}} =\]
\[= \frac{5^{- 3} \cdot 5^{4}b^{4}}{a^{7}} = \frac{5b^{4}}{a^{7}}\]