I want to know if i did the following calculation correctly and how can calculate the interest rate and where to enter it?

• a)If the ending cash balance as of June 30, 2017 was $10,000, determine the firm's foretasted monthly cash balance.
• b)The staff at The Right Target, Inc. wants to know how much they would need to borrow each month if the minimum ending cash balance is $30,000 and the annual interest rate is 7%.