How do you set up for excel and solve in excel creating a math formula and using an excel formula and creating a VBA formula how to solve this

I need to make a mathematical formula and i need to use the preset excel functions and create VBA. three formulas on each question but each question should have the same answer three times

4. Exactly six years ago, Cathy bought her dream home using a 30-year mortgage with an APR of 6.5% on a $500,000 loan. She has been making her monthly payments. Today, she came to know that her bank is offering a special mortgage refinance offer at an APR of 4.25% on 20-year mortgages. How much will Cathy's monthly payment change, if she decides to refinance today