In order to lift the stone off the floor onto a shelf that it's 1.5 m. high at a constant speed, it's needed to counteract the work done by gravity on the stone, so the total net work be zero, as there is no change in the kinetic energy during the process (except during the first instants when you take off the stone from the floor).
This work is just the negative of the product of the weight of the stone by the vertical displacement (as both are anti-parallel each other), which is equal to the change in the gravitational potential energy of the stone (taking the floor as the zero reference level) , as follows:
[tex]W_{g} = -\Delta U = U_{f} - U_{i} = m*g*h =- 5.6 kg*9.8 m/s2*1.5 m = -82.3 J[/tex]
