x³-11x²+31x-21 =x³-11x²+31x-20-1=x³-1-(11x²-31x+20)=(x-1)(x²+x+1)-(x-1)(11x-20)=(x-1)(x²+x+1-11x+20)=(x-1)(x²-10x+21)=(x-1)(x-3)(x-7)