Clint is building a wooden swing set for his children. Each supporting end of the swing set is to be an​ A-frame constructed with two 9​-foot-long 4 by 4s joined at a 45​° angle. To prevent the swing set from tipping​ over, Clint wants to secure the base of each​ A-frame to concrete footings. How far apart should the footings for each​ A-frame be?