(1)∵M=3x2-2xy+y2,N=2x2+xy+3y2,∴M+N=3x2-2xy+y2+2x2+xy+3y2=5x2-xy+4y2;(2)∵M=3x2-2xy+y2,N=2x2+xy+3y2,∴M-N=3x2-2xy+y2-2x2-xy-3y2=x2-3xy-2y2.