\[14a^{3} + 21ab^{2} - 4a^{2}b - 6b^{3} =\]
\[= 7a\left( 2a^{2} + 3b^{2} \right) - 2b\left( 2a^{2} + 3b^{2} \right) =\]
\[= \left( 2a^{2} + 3b^{2} \right)(7a - 2b).\]