How would I calculate the NPV of a 36 month lease at $342.86 payment per month with an interest rate of 8.5%?

In order to calculate the NPV(Net Present Value), check out this site.
Net present value - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Net_present_value)

But since you are referring to a lease payment on the car, I am not sure if this is what you are trying to find unless you are leasing the car to someone and they are paying you on a monthly basis. If so, you might try using Excel to calculate the NPV. It will give you all the information you need.