x^4-3x^3-3x^2+7x+6＝x^4-3x^3-3x^2+9x-2x+6＝x^3(x-3)-3x(x-3)-2(x-3)=(x-3)(x^3-3x-2)=(x-3)(x^3-4x+x-2)=(x-3){x(x^2-4)+(x-2)}=(x-3){x(x+2)(x-2)+(x-2)}=(x-3)(x-2)(x^2+2x+1)=(x-3)(x-2)(x+2)^2