Answer: 3 days in they will bot charge 39$
Explanation:
Biking shack= 21+6d(d for days)
Bike nation=12+9d
to figure it out lets plug in 1 day for each so for shack that is 29 dollars. for nation that is 21 dollars. now 2 days shack is 33. 2 days for nation is 30. now try 3 days and for shack that is 39 dollars. for nation it is also 39! so there is your answer, 3 days in and they will both cost 39 dollars, hope this helps! :)