I need help finding the interest rate on deposits that will maximize the bank's profit. Thank you.

19. Traditional banking depends on individuals depositing money and earning interest on their deposit at an annual interest rate of ry and the bank then lending that money at a (higher!) annual interest rate of ro to others. The bank needs to make ry big enough to get people to deposit their money with the bank, but it doesn't want to pay any more interest than it has to. From prior experience, a certain bank knows the amount of money deposited will be $200 million times ra, which it can then loan out to others at ro = 7% interest. For example, if the bank offers 3% interest on deposits (74 = 0.03), then the deposits will total 0.03 x 200,000,000 = 6,000,000 dollars. The bank will then take in $420,000/year in interest and pay out $180,000/year in interest, for a net profit of $240,000/year. What interest rate ry on deposits will maximize the bank's profit