-2ab^(y+1)与4a^xb是同类项，所以x=1,y+1=1,y=0.2x^2-3xy-4y^2-(3x^2-xy+9y^2)=2x^2-3xy-4y^2-3x^2+xy-9y^2=-x^2-2xy-13y^2=-1
