Question

factorize (x.x) -21x+108

Answer 1

x^2 - 21x + 108

= x^2 - 12 x - 9x + 108

= x ( x - 12 ) - 9 ( x - 12 )

= ( x - 12 ) ( x - 9 )

Answer 2

this is a quadratic equation in x,and then
x^2-21x+108
x^2-12x-9x+108
x(x-12)-9(x-12)
(x-12)(x-9)