\[4a^{3}b = - 5\]
\[a^{3}b = - \frac{5}{4}\]
\[4a^{6}b^{2} = 4 \cdot \left( a^{3}b \right)^{2} = 4 \cdot \left( - \frac{5}{4} \right)^{2} =\]
\[= 4 \cdot \frac{25}{16} = \frac{25}{4} = 6,25.\]