Explanation:
I'm assuming the sides can only be integers.
The most optimal area would be a square. We need to distribute 800 to 3 sides, which of course is not possible with integers. We will have to distribute the 800 as evenly as possible.
800/3 = 266.666666667.
We can let 2 sides be 267 and 1 side be 266. This will distribute it evenly. However, notices that the river side can be any length. Meaning that we can make one side be 268 and the other 2 sides be 266. This still satisfies our 800 feet of fencing, while being larger than 267 * 266.
Shorter side: 266 ft
Longer side: 268 ft
Greatest Possible Area: 71288