A baseball card store can obtain sports cards at a cost of $5 per card, and has been selling them at a rate of 50 cards per month for a price of $9. The store is planning to change the card price, and estimates that for each $0.50 per card reduction, 10 more cards will be sold each month. At what price per card will the store owner maximize his/her profit? I need to find the answer using calculus and I'm not sure how to solve it using calculus. If you could help me out I would appreciate it.