\[\frac{2}{7}a^{- 6}b^{4} \cdot \frac{21}{32}a^{8}b^{- 9} =\]
\[= \frac{2 \cdot 21}{7 \cdot 32}a^{2}b^{- 5} = \frac{3a^{2}}{16b^{5}}\]