x(x+1)(x-2)(x-3)-40
=x(x-2)(x-3)(x+1)-40
=(x^2-2x)(x^2-2x-3)-40
=(x^2-2x)^2-3(x^2-2x)-40
=(x^2-2x-8)(x^2-2x+5)
=(x-4)(x+2)(x^2-2x+5)