Question 258522
x = 3,   y = -3,   z = 1

Now for your attempt
(4x+2y+z=7)4
 2x+2y-4z=28
_____________
 2x+2y=-4
 16x+8y+28


(4x+2y+z=7) *4
16x+8y+4z=28
What happened to the 4z and why only three terms?
From here on it is trouble.
Let's try again


4x+2y+z=7 (1)
2x+2y-4z=-4 (2)
x+3y-2z=-8 (3)