The stone is initially at a height of 100 m from where it is dropped. The gravitational potential energy of an object with mass m at a height h from the ground is PE = m*g*h where g is the acceleration due to gravity .
At a height of 100 m from the ground, the potential energy of the 3 kg stone is 3*9.8*100 = 2940 J.
At a height of 50 m half its potential energy has been converted to kinetic energy. The potential energy of the stone is 3*9.8*50 = 1470 J.
The kinetic energy is 2940 - 1470 = 1470 J.
At a height of 50 m the potential energy of the stone is equal to its kinetic energy and both are equal to 1470 J.