Hey, I'm a student in finance and i don't understand well this question. Could you please explain to me? Thanks in advance

"A French businessman wants to take up Elon Musk's idea and build "tubes" under a large city, in which cars are thrown at high speed to reach the airport in a few minutes, avoiding traffic jams. The entrepreneur estimates that the initial investment will amount to 484 million, minus 403 million in combined aid from the government, the regional council and the city. In addition, the entrepreneur estimates that the net cash flow will be 8 million per year.

Having studied finance in business school, he wants to calculate the payback period while taking into account the time value of money. What is the payback period? That is, what is the "N" such that the NPV cancels itself out?

You will take 6.5% as the annual discount rate. That's the cost of capital to the business owner."