Along with your new car in five years, you have decided to buy a new house now. Current mortgage rates are 4% per year, and you have decided to finance the house for 30 years. The house you want is $860,000, and you are able to finance 80% of that. How much will your monthly payment be?

I know the formula but I do not understand what to do for the t or rate, also what do I do with the 80%?