\[ax^{6} - 3x^{6} - ax^{3} + 3x^{3} =\]
\[= ax^{3}\left( x^{3} - 1 \right) - 3x^{3}\left( x^{3} - 1 \right) =\]
\[= \left( x^{3} - 1 \right)\left( ax^{3} - 3x^{3} \right) =\]
\[= x^{3}(a - 3)(x - 1)\left( x^{2} + x + 1 \right).\]