Short Essay Question on Existential Freedom and the Aristotelian Account of Reality

How does Sartre's "existence precedes essence" reverse Aristotle's way of describing human beings?

(Break the question down into these parts):

How do Sartre and Aristotle differ on whether humans have an essence (or "entelechy")?
What are their arguments for adopting these views?
What objections can be raised against each view?
How would they reply to these objections?
What difference would it make to adopt each view?