=(x^2-4x-12)(x^2-4x-12+15)+56
= (x^2-4x-12)^2+15(x^2-4x-12)+56
= (x^2-4x-12+7)(x^2-4x-12+8)
=(x^2-4x-5) (x^2-4x-4)
=(x+1)(x-5)(x^2-4x-4)
有理数范围内只能这样了