A recent college graduate buys a new car by borrowing $18,000 at 8.4%, compounded monthly, for 5

Question:

A recent college graduate buys a new car by borrowing $18,000 at 8.4%, compounded monthly, for 5 years. She decides to pay an extra $15 per payment.
(a) What is the monthly payment required by the loan, and how much does she decide to pay each month?
(b) How many payments (that include the extra $15) will she make?
(c) How much will she save by paying the extra $15?