\[x^{4} - 10x^{2} + 9 =\]
\[= x^{4} - 9x^{2} - x^{2} + 9 =\]
\[= x^{2}\left( x^{2} - 9 \right) - \left( x^{2} - 9 \right) =\]
\[= \left( x^{2} - 1 \right)\left( x^{2} - 9 \right) =\]
\[= (x - 1)(x + 1)(x - 3)(x + 3)\]