The 3 sides of the triangle measure 5 feet 9 feet and 9 feet classify the triangle by its sides

Can somebody me out with this question

Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a certain commodity satisfy the demand equation 4p cubedplusx squaredequals38 comma 400. determine the rate at which sales are changing at a time when xequals80, pequals20, and the price is falling at the rate of $.20 per week.

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?