（3A-3B）-（2A+B）=3A-3B-2A-B=3A-2A-3B-B=A-4B=4x²-4xy+y²-4(x²+xy-5y²)=4x²-4xy+y²-4x²-4xy+20y²=4x²-4x²-4xy-4xy+y²+20y²=-8xy+21y²(2a+b)的过程2A+B=2(4x2-4xy+y2)+x2+xy-5y2=8x2-8xy+2y2+x2+xy-5y2=8x2+x2-8xy+xy+2y2-5y2=9x2-7xy-3y2