\[x^{2} - 10x + 21 =\]
\[= x^{2} - 10x + 25 - 4 =\]
\[= (x - 5)^{2} - 2^{2} =\]
\[= (x - 5 - 2)(x - 5 + 2) =\]
\[= (x - 7)(x - 3).\]