Eliminate z using equations 1 and 2: 4x + 7y = -17
Eliminate z again using equations 2 and 3: 6x + 7y = 3
Using the two new equations, eliminate y: 6x = 20
x = 20/6 = 10/3
Substitute x into one of the equations with only x and y:
4(10/3) + 7y = -17
40/3 + 7y = -17
7y = -17 - 40/3
7y = -51/3 - 40/3
7y = -91/3
y = -91/21
Substitute x and y into one of the original equations:
10/3 + 2(-91/21) - z = -7
10/3 - 182/21 - z = -7
70/21 - 182/21 - z = -7
-112/21 - z = -7
z = -7 + 112/21
z = -7 + 16/3
z = -21/3 + 16/3
z = -5/3