As a mathematics professor loads more weight in the back of his Subaru, the mileage goes down. Let $x$ be the amount of weight (in pounds) that he adds, and let
$y=f(x)$ be the mileage (in mpg).
a. Is $f^{\prime}(x)$ positive or negative? Explain.
b. What are the units of $f^{\prime}(x) ?$