Can anyone hep me with this problems? All three questions are related. Thank you in advance!

a. A man buys a car for $18,000 with no money down. He pays for the car in 30 equal monthly payments with interest at 12% per annum, compounded monthly. What is his monthly loan payment?

b. Right after 18 payments the interest rate dropped to 9% per annum, compounded monthly. What was the balance on the car loan right after he made the 18th payment?

c. Also s/he decided to make one lumpsum payment at the end of the loan term. Compute the balance on the loanfor the remaining 12 payments if s/he would want to make one single lumpsum amount at the end of the loan term.