Domain
The domain of 𝑓𝑔𝑥 is restricted to the elements of the domain of 𝑔 for which 𝑔𝑥 is in the domain of 𝑓.
So:
Pay attention to domain of inside function
Inside function has to be in the domain of outside function
Let 𝑓𝑥=𝑥 and let 𝑔𝑥=1𝑥−3.
Find an expression for 𝑓∘𝑔𝑥 
Find the domain of 𝑓∘𝑔𝑥