[1, 2, 3, -4]
[2, 1, 1, 1]
[3, 2, 2, 0]
[0, -1, -1, 3]
II - 2*I ; III - 3*I
[1, 2, 3, -4]
[0, -3, -5, 9]
[0, -4, -7, 12]
[0, -1, -1, 3]
3*III - 4*II ; 3*IV - II
[1, 2, 3, -4]
[0, -3, -5, 9]
[0, 0, -1, 0]
[0, 0, 2, 0]
c = 0
d = d
-3b + 9d = 0 --> b = 3·d
a + 2(3d) - 4d = 0 --> a = - 2·d
-2d * v1 + 3d * v2 + 0 * v3 + d * v4 = 0
-2 * v1 + 3 * v2 + 1 * v4 = 0