There are bunches of different ways to solve systems of equations.  
Which method are you studying?

7x −  y − z = 34
 x − 3y + z = 6
 x + 2y − z = 4

Here are 8 different solutions to that system.  If you substitute any one of
them, all three equations will be true.  

Solution 1:     x = 1   y = -8   z = -19
Solution 2:     x = 2   y = -6   z = -14
Solution 3:     x = 3   y = -4   z = -9
Solution 4:     x = 4   y = -2   z = -4
Solution 5:     x = 5   y = 0   z = 1
Solution 6:     x = 6   y = 2   z = 6
Solution 7:     x = 7   y = 4   z = 11
Solution 8:     x = 8   y = 6   z = 16

-------------------------------

4a − 3b = 1
6a − 8c = 1
2b − 4c = 0

Here's one solution to that system.  Substitute it and you'll get a true
equation:



Edwin

7x − y − z = 34 ..... equation (1)
x − 3y + z = 6  ..... equation (2)
x + 2y − z = 4  ..... equation (3)