hmmm... i don't see why did you do that ? from the start... i assume you have 3 equations ? express from 1st or third "z" and just put in another equations and solve them for x,y ...

from third ....

and put in first

and you'll get

can you continue from here ?

P.S. x =9 , y = 9 and z=6 are not the solutions .... x = -33/7 ... y = -69/7 ... z= 654/7 are the solutions of the system of equations