Since y is already eliminated from the middle equation, eliminate y
from the first and third equations by multiplying the third one by 3,
and adding term by term:

 2x - 3y -  9z = -24
-9x + 3y - 12z = -18
--------------------
-7x      - 21z = -42

Let's divide that through by -7

 x + 3z = 6

That's the same as the 2nd equation, so it is a dependent system.

Solve that for x:

x = 6 - 3z

Substitute that in either of the other two original equations.  I'll pick
the first one:

       -3x + y - 4z = -6
-3(6 - 3z) + y - 4z = -6
  -18 + 9z + y - 4z = -6
       -18 + 5z + y = -6
                  y = 12 - 5z

Solution:   (x, y, z) = (6 - 3z, 12 - 5z, z)

Edwin