Apendulum has 503 j of potential energy at the highest point of its swing. how much kinetic energy will it have at the bottom of its swing?
joules

Energy is conserved, the total energy never changes.
At the highest point, the velocity of the pendulum is zero. That is, the kinetic energy is zero. The total energy consists of potential energy only.
E = E pot = 503J
If we set the potential energy to zero at the lowest point and the total energy can not change, all the energy must have been converted into kinetic energy.
E = E kin = 503J.