A light, elastic string of natural length 2.4 m is stretched between two points, A and B, on a horizontal ceiling. The distance AB is 4m. The modulus of elasticity of the string is 60 N. A particle of mass 3.5 kg is attached to the midpoint of the string. The system rests in equilibrium, with both parts of the string making an angle of 30° with the ceiling.

a. Find the extension of the string.

b. Determine the amount of elastic potential energy stored in the string.