Given the function how can I prove that ?

What do YOU think ?
While we're happy to HELP we wont do all the work for you.
Show us what you have done and where you are having problems..

That's the problem, that I couldn't do anything. I don't know how to express the formula for endomorphism. I only know that the domain should be equal with the codomain and in this case is true, but that's all? I only need the hints for solving it, not the entire solution.