(x-4)(x+1)(x+7)(x+2)+81=(x+1)(x+2)(x-4)(x+7)+81=(x^2+3x+2)(x^2+3x-28)+81=(x^2+3x+2)(x^2+3x+2-30)+81=(x^2+3x+2)^2-30(x^2+3x+2)+81=(x^2+3x+2-27)(x^2+3x+2-3)=(x^2+3x-25)(x^2+3x-1)