you've got a system of 3 simultaneously equations:
I don't know which method you use to solve this system of equations. I solved the EQU1 for y and plugged this term into EQU2. I got:
3x + 5z = 41
-15x + z = -71
Multiply the 1rst equation by 5 and add:
26z = 134. Solve for z. Re-substitute this value and solve for x. Re-substitute the value of z in the EQU2 and solve for y.
The sum of these three numbers is 12