Thanks Damon! first I eliminated z by adding equation 1+2 (1st one *2) and got same as urs x+y= 10. Then, I eliminated z by adding equation 2+3 (2nd equation*3, and 3rd equation*2) and got x-y=6.

x+y= 10
x-y=6
x=16
why is this wrong?

x+y= 10

x-y=6
---------------
2 x=16
x = 8