Gravitational potential energy of a 3.0 kg book raised 2.0 m

Assumptions: the height is measured relative to the starting level and g = 9.8 m/s^2.
Use U_g = mgh.
U_g = (3.0 kg)(9.8 m/s^2)(2.0 m)
U_g = 58.8 kg*m^2/s^2
Since 1 J = 1 kg*m^2/s^2, U_g = 58.8 J.