To do elimination, you combine the two equations by adding
-9x + y = 16
+ x - y = 0
-8x + 0y = 16
Now that there are 0y, we can solve for x:
-8x = 16
divide both sides by -8
x = -2
To solve for y, plug the answer you got for x in to the original equations.
-9x + y = 16
-9(-2) + y = 16
18 + y = 16
y = -2
x - y = 0
-2 - y = 0
y = -2
x = -2 and y = -2
