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.