Tristan has this rectangular bulletin board shown. What is the most she can increase the length of her bulletin board in order to maintain a perimeter of no more than 21 feet?
A. 3ft
B. 3.5ft
C. 6ft
D. 7ft
![Tristan has this rectangular bulletin board shown What is the most she can increase the length of her bulletin board in order to maintain a perimeter of no more class=](https://us-static.z-dn.net/files/d74/e8571b555d07a4a1c4f4c6fa211bd3a9.jpg)