Domain and Range Functions help!?

The problem is that I have this math cl that's killing me. So, if x + y = 4; how do I solve this problem if there's no f(x) = x+ #. I understand if f(x) = x + 2 because if x=1 then f(1) = x + 2. Plug (1) where the x would be, 1 + 2 = 3. But the one I have is so way out there, I cannot figure it out. (Not to mention I think I have Swine Flu and can't concentrate right now!) Can anyone help me figure this out? Am I in the right area of this answer? Thanks for all your help! Oh yes, I have to solve this and justify my answer.