You may want to review functions first.

One property that a function can have or not have is to
be *one-to-one*. That just means that no second member
appears more than once. (Remember, if a *first* member
appears more than once, then it's not a function at all!) For
example, {(−5 ⁄ 3, 1),
(−3 ⁄ 8, 5 ⁄ 7), (1,
−6), (8, −1 ⁄ 3)} is one-to-one,
but {(−2, −2 ⁄ 7),
(−5 ⁄ 8, 5 ⁄ 6),
(−1 ⁄ 2, −1),
(−3 ⁄ 10, −1), (0, 2), (3,
1 ⁄ 5), (4, 0)} is not, since −1 appears
twice as a second member.