Question 1091455
 x + y + z = 12
2x + y - z = 20
-x - y + 4z = - 17
 x + y + z = 12
------------------------- Add eqn 1 & 3
5z = -5
z = -1
---------
 x + y + z = 12
2x + y - z = 20
================
 x + y = 13
2x + y = 19
---------------

Subtract
-x = -6
x = 6
----------------
 x + y + z = 12
 6 + y - 1 = 12
y = 7
--> intersection at (6,7,-1)