\[x^{2} - 12x + 20 =\]
\[= x^{2} - 12x + 36 - 16 =\]
\[= (x - 6)^{2} - 4^{2} =\]
\[= (x - 6 - 4)(x - 6 + 4) =\]
\[= (x - 10)(x - 2).\]