So, imagine the trainer is in the very middle of the circle and he is holding out a rope that's attached to the horse. The rope here from the trainer to the horse represents the radius of the circle. The horse trots around the trainer (middle point) in a circle.
The formula to find area of a circle is A = pi*r^2 (pi times r squared where r is radius)
We know that the radius of the "circle" here formed as the horse trots around the trainer is 31 (the 31 foot long rope).
So let r = 31 & plug the value into the formula to find circle area.
A = pi*31^2 = 3019.07054
The area is therefore 3019.07 square feet. (rounded)