Work done=force *displacement
mg * height
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Potential energy of a body is the work done to take the body to a particular height. Let a body having mass m is raised to a particular height h, then the gravitational force acting on the body of the earth is m*g (which acts downwards). So the work done against he gravity will be-
Work done = F*s , where F stands for Gravitational force and s stands for displacement
Work done = mg*h
Work done = mgh
Potential Difference = mgh 
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ur welcome!! :)
pls help me in another one..