(1){4x-9z=17 1 {3x+y+15z=18 2 {x+2y+3z=2 3(2){2x+4y+3z=9 1 {3x-2y+5z=11 2 {5x-6y+7z=13 3题目末尾的1,2,3表示几式要完整过程想了半天没想出来