Exercise 16 Page 65

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 graph, we can check if each $x\text{-}$value has only one $y\text{-}$value by subjecting it to the Vertical Line Test.