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