Answer:
or

Explanation:
I could be 100% wrong ( I solved the problem the way it looks like it should be, but I don't recognize this way of writing one of these) but here we go:

A little explanation:
Consider that f(x) is equal to what they tell you it is. With that in mind, f(x)-x would most likely be what they told you f(x) was with a "-x" slapped onto it.
Also:
because any number divided by itself is one so 3/3=1 and 3x/3=1x=x
Hope I could help :)