用初等行变换把下列矩阵化为行最简形矩阵

1 0 2 -1
0 0 -1 3
0 0 -2 6

1 0 2 -1
0 0 -1 3
0 0 0 0

1 0 2 -1
0 0 -1 3
0 0 0 0

1 0 2 -1
0 0 1 -3
0 0 0 0

1 0 0 5
0 0 1 -3
0 0 0 0