Exercise 13 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.

$$\begin{array}{c}(3,\text{-}9),(11,21),(121,34),(34,1),(23,45)\end{array}$$

Examining the ordered pairs, we can see that there is no $x\text{-}$value with multiple $y\text{-}$values. Therefore, the relation is a function.