M－N=3x^2-2xy+y^2-2x^2-xy+3y^2=x^2-3xy+4y^2M+N=3x^2-2xy+y^2+2x^2+xy-3y^2=5x^2-xy-2y^2
M－N=X^2-3XY+4Y^2M+N=5X^2-XY-2y^2
