Exercise 15 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.

On the given graph, there is no visible part where two points have the same $y\text{-}$value. Each vertical line passes through the graph at only one point, so the graph represents a function.