Using the payment formula and her handy calculator, the dealer determines that your monthly payment will be $367.31. Thus the pair of numbers (19000, 367.31) is one of the pairs
(price of car, monthly payment)
that make up the function defined by the payment formula when D = 2700, r = 0.00325, and n = 48.
Unfortunately, you cannot afford a monthly payment of $367.31, so you will have to shop around to find a car you can afford. In this situation, the monthly payment is no longer the independent variable. Now the price you can afford depends on the monthly payment you can afford, and the roles of independent and dependent variables have been reversed. In effect, there is a new function involved in this problem, namely, the pairing relation
(monthly payment, price of car).
This second function has a special — and obvious — relationship to the first function. The two functions are said to be inverses of each other.
When you substitute p = 280 in the payment formula, along with D = 2700, r = 0.00325, and n = 48, you can then solve for P to find out what price car you can afford. You should have found that the most expensive car you can afford costs about $15,125.