x+2y=9
2x-y=8

Note that the y term in the first equation is positive and negatve in the second, but they differ by a factor of 2. So if you multiply the second equation by 2 you can then add it to the first equation anf cancel out the y's, like this:

x + 2y = 9
+ 2(2x-y) = 2(8)
-----------------
x + 2y + 4x -2y = 9 + 16,

Regroup:
5x = 25

Can you take it from here?