I want the full step about how to calculate part a+b, thanks.

2. You are bullish on HKEx stock. The current market price is $250 per share, and you have $100,000 of your own to invest. You borrow an additional $100,000 from your broker at an interest rate of 5% per year and invest $200,000 in the stock. a. Suppose the price of HREx stock falls immediately after your purchase. The maintenance margin is 35%. How low can the price of HREx stock fall before you receive a margin call? b. Suppose a year has passed. What is your rate of return if the price of HKEx stock has gone up by 20%? Ignore the expected dividend