The question is exactly as appeared in the math book. I'm sure it is another plug and chug, right?
I just get confused by the negative in front of f(x). See it?
What do I do in this case?
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.