I believe the answer is 4.8. A rectangle has 4 sides, two pairs. Each pair has the same length, so if there is one 3.6 in. side, there is another. So subtract (3.6•2=)7.2 from 16.8, and you get 9.6. divide this by two because the remaining 2 sides must be the same, and you get 4.8.
Hope this helps!
