Đáp án:
$1) a) M - (x^2y-1) =-2x^3+x^2y +1$
$⇔M -x^2y+1 =-2x^3+x^2y+1$
$⇔M = -2x^3+x^2y+x^2y+1-1$
$⇔M= -2x^3+2x^2y$
Vậy $M=-2x^3+2x^2y$
$2) b) 3x^2+3xy-x^3 -M =3x^2+2xy-4y^2$
$⇔M = 3x^2+3xy-x^3 - (3x^2+2xy-4y^2)$
$⇔M =3x^2+3xy-x^3 -3x^2 -2xy +4y^2$
$⇔M = -x^3 +xy +4y^2$
Vậy $M =-x^3+xy+4y^2$