Multi-step trig. problems

Because of your favorite lemonade it is $3.84 for 3 gallons. write this as an unit rate

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.

How many $20 bills do you need to have to make $280