Age of drivers in crash fatalities the number of crash

Question

Question: Age of drivers in crash fatalities the number of crash fatalities per 100,000 vehicle miles of travel (based on 1994 data) is approximated by the model

f(x) =                 15                                (0 ≤ x ≤ 11)
        0.08333x² + 1.91667x + 1

Where x is the age of the driver in years, with x = 0 corresponding to age 16. Show that f is decreasing on (0, 11) and interpret your result.