What should be the height of the building when a 20 kg objectdropped fromthe top of the building possesses 5000j of energy at the time of fall?

When you say "time of fall", I'm going to assume you mean "time of hitting the ground".

The kinetic energy it has when it hits the ground is exactly the potential energy

it had on the roof. (As is our tradition, we're ignoring air resistance.)

Potential energy = (mass) x (gravity) x (height)

5,000 = (20) x (9.8) x (Height)

Height = 5,000 / (20 x 9.8) = 25.51 meters