Find the length of a rope by which a cow must be tethered

Find the length of a rope by which a cow must be tethered in order that it may be able to graze an area of 154 sq m.

Length of to the rope = Radius of circle
According to the question,
πr² = 154
∴ r² = 154 x (7/22) = 7 x 7 = 49
∴ r = √49 = 7 m