(x^2+2x-10)(x^2+2-18)-20
=(x^2+2x-10)(x^2+2x-10-8)-20
=(x^2+2x-10)^2-8(x^2+2x-10)-20
=(x^2+2x-10-10)(x^2+2x-10+2)
=(x^2+2x-20)(x^2+2x-8)
=(x^2+2x-20)(x+4)(x-2)