x^3-X^2-10X^2+31X-21 =X^2(X-1)+(10X-21)(-X+1)=(X-1)(X^2-10X+21)=(X-1)(X-3)(X-7)
=(x^3-10x^2+21x)-(x^2-10x+21)=(x-1)(x-3)(x-7)
(x-1)(x-3)(x-7)
