The potential energy of the object is going to be gravitational: PE=mgh. Assuming we're talking about the gravitational potential energy relative to the bottom of the cliff, the object's height is 30 m. g is 9.8 m/s^2. We don't know its mass. You could technically use the equation F=mg to find the mass, where F=40 and g=9.8, but that's unnecessary. You can just substitute F into PE=mgh to get PE=Fh. Substitute the given values to get PE=40*30=120 J.
