Solve the following system:

x + 2y  = 4    (1) 

2x - y  = -7  ⇒   y  = 2x + 7   ( 2)

Sub (2) into (1)  for y

x + 2 [2x + 7]  = 4

x + 4x + 14  = 4

5x + 14  = 4      subtract 14 from both sides

5x = -10            divide both sides by 5         

x = -2

So....using (2).....y  = 2(-2) + 7  =  3

So....the last equation becomes

-2 + 3 + z  = - 4

1 + z  = -4      subtract 1 from both sides

z  = - 5

{x, y, z }  =  { -2, 3, - 5}