Factorise `x^(2)-21x+108`.

Explanation

Given expression : `x^(2)-21x+108`
Here, constant term is +108.
Now, we take two factors of 108 whose sum is `-21`.
Such factors are -12 and -9.
`therefore` Given expression : `x^(2)-21x+108`
`=x^(2)+(-12-9)x+108=x^(2)-12x-9x+108`
`=x(x-12)-9(x-12)=(x-12)(x-9)`