- w+x+y+z=1
- w+x-y-z=7 is the same as 2w+2x-2y-2z=14
- w-2x+2y+z=9
- w-x+7y+5z=-15
Add (1) and (2): 2w+2x=8, that is, w+x=4, w=4-x.
Subtract (2) from (1): 2y+2z=-6, that is, y+z=-3, z=-3-y=-(3+y).
By substituting for w and z in (3):
(4-x)-2x+2y-(3+y)=9 simplifies to -3x+y=8 (5); and in (4):
(4-x)-x+7y-5(3+y)=-15, 4-2x+7y-15-5y=-15, -2x+2y=-4, -x+y=-2, y=x-2.
By substituting for y in (5):
-3x+(x-2)=8, -2x=10, x=-5, and y=-7, w=9, z=4 from other equations in bold print.