Suppose that a when a cell phone store charges $16$$ for a car charger, it sells $63$ units. When it drops the price to $15$$ it sells $66$ units. Assume that demand is a linear function of price. If each phone charger costs $1$$ to make, what price should the store charge to maximize its profit?

If $x$ is the number of times the price is reduced by one dollar. Find a function for total profit with respect to $x$. A negative value for $x$ will mean the price is increased.
$f(x) =$

Price to maximize profit $=$