A function is a relation for which each value from the set the first components of the ordered pairs is associated with exactly one value from the set of second components of the ordered pair.
(2, 0), (-3, 3), (9, 1), (-3, 5) NOT
(9, 1), (-3, 4), (2, 1), (9, 2) NOT
(2, 4), (-3, 2), (9, 1), (-7, 2) YES
(2, 4), (-3, 6), (2, 3), (-7, 2) NOT