How do i answer this showing my work?

Your grandparents have moved into assisted care living. They don't want to sell their house just yet, but you know they'll never meve back into it. Still, to honour their wish, you decide not to sell the house but to lease it for tifteen years, using the income to help pay for their assisted care. The lessees are to pay you $24,000 annually, with the first payment due immediately. However, much to your surprise the lessees offer to pay you a single lump sum upfront for the 15-year lease. They offer you $270,000! If you accept the deal, what interest rate will be implied in the deal