Gravitational potential at a distance r from the centre of the earth is given by, V=r−GM.
Therefore, we can write
−RE+hGME=−5.12×107…(i)
also acceleration due to gravity at that point will be
(RE+h)2GME=6.4…(ii)
By (i) and (ii), we get
(RE+h)2GME−RE+hGME=6.45.12×107
⇒RE+h=80×105m=8000km⇒h=8000km−6400km=1600km