x^3-11x^2+31x-21=x^3-11x^2+10x+21x-21=x(x^2-11x+10)+21(x-1)=x(x-1)(x-10)+21(x-1)=(x-1)[(x(x-10)+21]=(x-1)[x^2-10x+21]=(x-1)(x-3)(x-7)
