Mortgage Excel Function

I’m trying to create the mortgage calculator through Excel for my assignment. For the Normal Mortgage, with the output is Present Value of a Loan amount, i use this formula:

PVoa = PMT [(1 - (1 / (1 + i)n)) / i]

where :
PVoa = Present Value of an Ordinary Annuity
PMT = Amount of each payment
i = Discount Rate Per Period
n = Number of Periods

What if I have PMT, n, PVoa as the inputs and i want to calculate the interest rate (i) ? Which function in Excel helps me to calculate (i) ?

Thanks a lot for your suggestions.
Hi Alfonso,
Could u please be more specific ?
Say, I already got these 3 input : payment, present value of a lump sum, numbers of payments ( years * frequency), what function in Excel i should use to calculate interest ?