Exercise 15 describes a regression model that estimates a cereal’s potassium content from the amount of fiber it contains. In this context, what does it mean to say that a cereal has a negative residual?

Data From Exercise 15

For many people, breakfast cereal is an important  source of fiber in their diets. Cereals also contain potassium, a  mineral shown to be associated with maintaining a healthy blood pressure. An analysis of the amount of fiber (in grams)  and the potassium content (in milligrams) in servings of 77 breakfast cereals produced the regression model Fiber. If your cereal provides 9 grams  of fiber per serving, how much potassium does the model estimate you will get?

Potassium 38+ 27.