To make your formula, you just need to determine the effective rate of 9% compounded continuously. To refresh your memory, by effective rate, I am of course referring to the nominal rate compounded annually. Once you have determined that, you merely plug that rate to the standard present value annuity formula(s), i.e. beginning of year payment version or end of year payments version, depending on what you mean by $500 per year for 10 years.