9.5 ft.
Perimeter means it adds all the sides. So length is 12.4 feet. There are two sides so 12.4 times 2 equals 24.8.
The perimeter is 43.8 so we are missing two sides of the width. 9.5 is the accurate answer because 9.5 times 2 is 19, and add 19 with 24.8 you get 43.8.