去括号,答案应该是3x+2y.先紧括号里的做,顺序是小括号中括号大括号.正负得负,正正得正,负负得正
=2x-{-3y+[2x-3x+y]}
=2x-{-3y+y-x}
=2x-{-2y-x}
=3x+2y