(2,-1,-1,1,2;1,1,-2,1,4;4,-6,2,-2,4;3,6,-9,7,9)怎样化为行阶梯形矩阵

r3-2r1,r1-2r2,r4-3r2
0 -3 3 -1 -6
1 1 -2 1 4
0 -4 4 -4 0
0 3 -3 4 -3
r3*(-1/4),r1+3r3,r4-3r3
0 0 0 2 -6
1 1 -2 1 4
0 1 -1 1 0
0 0 0 1 -3
r1-2r4
0 0 0 0 0
1 1 -2 1 4
0 1 -1 1 0
0 0 0 1 -3
交换行
1 1 -2 1 4
0 1 -1 1 0
0 0 0 1 -3
0 0 0 0 0