can someone please answer all parts please

Kobe has a mortgage for $478,353.00. The term of the mortgage is 4 years, and the amortization period is 15 years. Kobe will make monthly payments and the mortgage rate is r(365) = 3.250%. The bank offers Kobe a flexible repayment plan, so once a year he can miss a payment or make a payment twice as large as usual. Each year, Kobe misses the first payment of the year. a) How much would Kobe owe at the end of the mortgage term if they didn't miss any payments? $ b) How much does kobe still owe at the end of the mortgage term (including the extra/missed payments)? $