The triangle inequality applies.
In order for ACD to be a triangle, the length of AC must lie between CD-DA=0 and CD+DA=8.
In order for ABD to be a triangle, the length of AC must lie between BC-AB=3 and BC+AB=9.
The values common to both these restrictions are numbers between 3 and 8. Assuming we don't want the diagonal to be coincident with any sides, its integer length will be one of ...
{4, 5, 6, 7}
13 kids sandwiches X 4 inches = 52 inches
22 small sandwiches X 6 inches = 132 inches
17 medium sandwiches X 8 inches = 136 inches
17 large sandwiches X 12 inches = 204 inches
52+132+136+204= 524 inches = 43.66 feet = 43.7 feet
