How to solve system of functions?

I obviously don't know the exact problem, but was it asking you about the range where a function was defined? For example, a function might be defined like this:

f(x) = 2/x for 3 < x < 10

That function is only defined on the range of numbers between 3 and 10. If you give it a value for x that isn't in that range, you can't determine the value of the function (it's undefined.)

Now, suppose you had another function, maybe this one:

g(x) = x + 1

They could then ask you, what's the range where f((g(x)) is defined? The answer would actually be 2 < x < 9. That's the range that would give you values between 3 and 10 for g(x), which are the values you can then feed to f(x).