Could someone help me please how to solve them with the normal formulas and not the excel formulas?

3. You are considering taking out a 20 year xed rate mortgage on a $640,000 house. Current mortgage rates are 3.5% (nominal rate, compounded monthly). What is the remaining principal after two years (i.e., immediately after the 24th payment)? What are the total interest payments in the first two years? After exactly two years (i.e., immediately after the 24th payment), you decide to use a $150,000 bonus you received at work to reduce the principal outstanding on your mortgage. You continue making the same monthly pay-ment that was calculated in part (a). When will the loan be fully paid off