The potential energy of an apple is 6.00 the apple is 3.00 meter high. What is the mass of the apple?

Potential Energy = Mass x Gravitational Constant x height = mgh

6 = m (9.81) (3)

Divide both sides by 9.81 and 3 to get m alone.

6 / (9.81 x 3) = 0.204

Also, don't forget units! I'm not sure what your units are for the mass because you didn't give units for Potential Energy.