It is said that Galileo dropped objects off the Leaning Tower of Pisa to
determine whether heavy or light objects fall faster. If Galileo had dropped a
5.0-kg cannon ball to the ground from a height of 12 m, what would have
been the change in PE of the cannon ball?

i will answer your question if you answer mine

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.