Answer:
Two x's can have the same y, but an x cannot have different y's. For example, [(2,3)(-2,4)(3,3)(5,18)] is a function because all of the x's have a specific y, and there is not more than one y to an x. However, [(2,3)(-3,7)(2,-4)] is not a function because one of the x's has more than one y.
Explanation:
(hope this helps-you can just put the first sentence...the rest is kinda wordy :P)