First find the radius of the circle:
The formula for circumference = C = 2 x PI x radius
The circumference would be the length of the fence.
100 = 2 x 3.14 x radius
100=6.28 x radius
Radius = 100/6.28
Radius = 15.92
Area of a circle = PI x radius^2
Area = 3.14 x 15.92^2
Area = 795.82 square feet.