r2-r1,r4-r1
1 2 0 1
0 1 5 -1
0 1 5 6
0 0 3 3
r3-r2
1 2 0 1
0 1 5 -1
0 0 0 7
0 0 3 3
r3r4 (行列式变符号)
1 2 0 1
0 1 5 -1
0 0 3 3
0 0 0 7
= -21.