Density=mass/volume
Your volume is 0.5*0.5*0.5, which is 0.125m^3, andd you have the density (i'm not sure if you typed 2.0*103 or 2.0*10^3).
Anyway, potential energy=mass*gravitational pull*height
So you need to find mass
Rearrange the density formula
Density=mass/volume
Density*volume=mass
Once you find the mass, multiply it by 9.81 (gravitational pull) and 3m (height) to find potential energy
