Proving Two Functions Are Inverses
You must show that 𝑓∘𝑔𝑥=𝑥 and 𝑔∘𝑓𝑥=𝑥 
(note: beyond this being a technique to show the functions are inverses, this is the definition of inverse functions)
Example:
Show 𝑓𝑥=4𝑥+9 and 𝑔𝑥=𝑥−94 are inverses.