Aball is dropped from the roof of a building. the mass of the ball is 3.0 kg. what is the potential energy of the ball in the instant that it is exactly 18.0 meters above the ground?

According to given condition there is no height(m) given from roof of building to the ground, there height given 18 m at a point above the ground.                         So, h=18m  ,                mass=3kg     ,         g=9.8m/s2                                                                      P.E=mgh                                                                                                                P.E=(3)(9.8)(18)                                                                                                      P.E=529J