\[x^{12} - 6x^{10} + 9x^{8} - 36 =\]
\[= x^{8}\left( x^{4} - 6x^{2} + 9 \right) - 36 =\]
\[= x^{8}\left( x^{2} - 3 \right)^{2} - 36 =\]
\[= (x^{6} - 3x^{4} - 6)(x^{6} - 3x^{4} + 6)\]