For the composite function f^2(x) how would you approach this?

f^2(x) is a shorthand way of writing f(f(x)). So for example if f(x) = 2x+3 then f(f(x)) gives you 2(2x+3) + 3. You take the original function f(x) = 2x+3 and then you replace the x in it with f(x) again. So here the original x has been replaced by 2x+3, which is f(x).

Once you have done that you can simplify it if you need to and it is possible to do so.