A 40 x 110 rectangle has an area of 4400 sq. ft.
The horse has a 14 ft lead. That makes 14 ft. the radius that the horse can cover.
The formula for the area of a circle is r^2 * PI ( radius squared * 3.14 ).
So the horse can't get to 4400 - ( 14 * 14 * 3.14 ) sq. ft.