From the question,
Horse is tied to a pole, then the pole will be the central point and the area over which the horse will graze will be a circle. The string by which the horse is tied will be the radius of the circle.
Then,
Radius of the circle, r = length of the string = 21 m
Then, area of the circle = πr2
= (22/7) × 21 × 21
= 22 × 3 × 21
= 1386 m2
Hence, the area of the horse can graze is 1386 m2.