\[5a^{2}b = 8 \Longrightarrow \ \ a^{2}b^{3} = \frac{8}{5}.\]
\[0,5a^{6}b^{9} = 0,5 \cdot \left( a^{2}b^{3} \right)^{3} =\]
\[= 0,5 \cdot \frac{512}{125} = \frac{256}{125} = 2\frac{6}{125}.\]