Upon starting a new job, you take out a $27,000 loan to buy a new car. The loan is scheduled to be paid off in 5 years over 60 equal monthly installments. The annual interest rate on the loan equals to 3.6% APR.

a. What is the required monthly payment of the loan?

b. Three years later, along with 36th monthly payment, you decide to make an additional one-time payment (amount X) to pay off the loan completely. Calculate X.