show fomula please

Prairie RV Mart offers you the following deal. You can pay $36,999 cash for a 2013 Grand RV and drive it off the lot. Alternatively you can make a 10% down payment today and then pay "only" $950 at the end of every month for the next four years (i.e. $3699.90 today followed by 48 payments of $950). What is the monthly compounded APR implied in the second payment option? You can use the IRR function, Goal Seek, or Solver in Excel to solve this problem. 4