please show steps of calculations. im trying to learn this

Dave just took out a variable-rate mortgage on his new home. The mortgage value is $150000, the term is 20 years, and initially the annual percentage rate is 6% with monthly compounding. The interest rate is guaranteed for 4 years, after which time the rate will be adjusted according to the prevailing rates. The new rate can be applied to the loan either by changing the payment amount or by changing the length of the mortgage. 1) What is the original monthly mortgage payment? (Assuming mortgage payments are monthly) 2) What will be the mortgage balance after 4 years? 3) If the interest rate on the mortgage changes to 9% after 4 years, what will be the new monthly payment that keeps the termination time the same? 4) Under the interest change in 3), what will be the new termination time if the payments remain the same