x^4+7x^3+8x^2-13x+15 =x^4+2x^3+5x^3+10x^2-2x^2-13x+15 =x^2(x^2+2x)+5x(x^2+2x)-2x^2-13x+15 =3x^2+15x-2x^2-13x+15 =x^2+2x+15 =3+15 =18
