>>41
3x^2+(y+6)x-(2y-3)(y+1)

3  -(2y-3) -2y+3
 X           y+6
1  (y+1) 3y+3

3  (2y-3) 2y-3
 X           -y-6
1  -(y+1) -3y-1

3  (y+1) y+1
 X           -5y-8
1  -(2y-3) -6y-9

3  -(y+1) -y-1
 X           5y+8
1  (2y-3) 6y+9

以上から

(3x-(2y-3))(x+(y+1))