Considering that the potential energy of the block at the table is mgh/3 and that on the floor is −2mgh/3, what is the change in potential energy δu of the block if it is moved from the table to the floor?

Explanation:

The change in potential energy is the difference between the energy at the floor level and the energy at the level of the table:

Given these two points and an unknown m and h, the change in the potential energy due to the "fall" from table to the floor is -mgh