the negative of the remainder to the original polynomial… or, rearrange the polynomial to feature as a factor x2+2x−3x
x4+2x3−2x2+x−1=x4+2x3–3x2+x2+2x−3−x+2=
=x2(x2+2x−3)+(x2+2x−3)−x+2=(x2+1)(x2+2x−3)−x+2
So it looks like if you add x−2 to your original polynomial, it becomes a multiple of x2+2x−3