A used book store buys a hardback book for $1.50 and then sells it for $5. Over time, the store sells the same number of books it buys. The store manager can use the
equation P(x) = 5x - 1.5x to determine the store's profit, P(x), where x is the
number of books that the store sells. Which statement about the book store is true
based on the profit equation?