Okay, so this is a problem that I have absolutely no idea how to start. Other than making random conjectures or saying "These are all integers!" I have no idea how to go about solving this. A pointer on where to start would be greatly appreciated.

http://img29.imageshack.us/img29/2483/helpa.png

We're given the hint to prove that each element of the above set solvesn = ax +bybut I don't know how to go about doing that!