Let p = x^2-7x

The original expression is converted to

(x^2-7x)^2+10(x^2-7x)-24

=p^2+10p-24

=(p+12)(p-2)

Substitute p and continue factoring

(x^2-7x+12)(x^2-7x-2)

=(x-3)(x-4)(x^2-7x-2)