There is a built in function from the Microsoft.VisualBasic assembly. I can use it in VB like this:
Financial.Pmt((dAPR / 100) / 12, iNumberOfPayments, dLoan
You must add a reference to the Microsoft.VisualBasic.dll assembly for your project in Visual Studio. This is not the same thing as the using Microsoft.VisualBasic; directive at the top your source file. You must do both steps.
using Microsoft.VisualBasic;