\[5a^{3} + 10ab^{2} - 4a^{2}b - 8b^{3} =\]
\[= 5a\left( a^{2} + 2b^{2} \right) - 4b\left( a^{2} + 2b^{2} \right) =\]
\[= \left( a^{2} + 2b^{2} \right)(5a - 4b).\]