x²+xy-6y²=（x+3y)(x-2y)
x^3-3x^2+4
=x^3-2x^2-x^2+4
=x^2(x-2)-(x+2)(X-2)
=(x-2)(x^2-x-2)
=(x-2)(x+1)(x-2)
=(x-2)^2(x+1)