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Acircle has a diameter will endpoints of (-2,8) and (6, 4). what is the center of the circle

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.

Could somebody me with this radical simplification?