Final answer:
The pendulum has 50J of potential energy at its highest points during its swing, which are on either side of the rest position.
Step-by-step explanation:
The pendulum has 50J of potential energy at the two points of its swing where all of its energy is potential and none is kinetic. This occurs at the highest points in its swing, on either side of its vertical rest position. By using conservation of energy, it's known that the potential energy at the top converts to kinetic energy at the bottom. At the points of maximum displacement, kinetic energy is zero, and all the energy of the system is potential.