1.
[3, 2, 6, a]
[4, 5, 12, b]
[2, 2, 5, c]
II' = 4*I - 3*II
III' = 2*I - 3*III
[3, 2, 6, a]
[0, -7, -12, 4·a - 3·b]
[0, -2, -3, 2·a - 3·c]
III' = 2*II - 7*III
[3, 2, 6, a]
[0, -7, -12, 4·a - 3·b]
[0, 0, -3, - 6·a - 6·b + 21·c]
I' = I + 2*III
II' = II - 4*III
[3, 2, 0, - 11·a - 6·(2·b - 7·c)]
[0, -7, 0, 28·a + 21·(b - 4·c)]
[0, 0, -3, - 6·a - 6·b + 21·c]
I' = 7*I + 2*II
[21, 0, 0, - 21·(a + 2·(b - 3·c))]
[0, -7, 0, 28·a + 21·(b - 4·c)]
[0, 0, -3, - 6·a - 6·b + 21·c]
[1, 0, 0, -a - 2b + 6c]
[0, 1, 0, - 4a - 3b + 12c]
[0, 0, 1, 2a + 2b - 7c]
x = -a - 2b + 6c
y = - 4a - 3b + 12c
z = 2a + 2b - 7c