Question 53279
1st: x+y=9
2nd: x-y=9 
Add the two equations to get:
3rd: 2x=18
x=9
Substutute that into 1st to solve for y, as follows:
9+y=9
y=0
Check the answer in 2nd.
x-y=9
9-0=9
9=9
Cheers,
Stan H.