Exercise 3 Page 156

For a relation to be a function, each $x\text{-}$value can only be paired with one $y\text{-}$value, but one $y\text{-}$value can be paired with multiple $x\text{-}$values. In a mapping, the $x\text{-}$values in the input can only point to one $y\text{-}$value in the output.