f(2) = 16, Let this be represented as (2, 16)
f(4) = -2, Let this be represented as (4, -2)
[tex]\begin{gathered} \text{slope =}\frac{change\text{ in y}}{\text{change in x}} \\ \\ \text{slope = }\frac{-2-16}{4-2} \\ \text{slope = }\frac{-18}{2} \\ \\ \text{slope}=\text{ -9} \end{gathered}[/tex]