Hi, I've been trying to solve this for over an hour now by extending the method of elimination like the textbook says. Very frustrated.

Please help.

Solve each systems. Express the solution in the form (x,y,z,w).

x+y+z-w=5

2x+y-z+w=3

x-2y+3z+w=18

-x-y+z+2w=8

The answer is (2,1,5,3)

I don't know how to get there.