Find -f(x) for:
(a) f(x) = 3 - x
(b) f(x) = 1/x
What does it mean when you have "-3" for example? I think some of the problem here is in the way you are looking at "f(x)." In the context you are looking at it f(x) is just a (probably real) number. So the negative sign is the same as it is in any other context when applied to a number.
-Dan