\[x^{7} - x^{3} = x^{3}\left( x^{4} - 1 \right) =\]
\[= x^{3}\left( x^{2} - 1 \right)\left( x^{2} + 1 \right) =\]
\(= x³(x - 1)(x + 1)(x^{2} + 1)\)