\( = \left( {\frac{{ - GMm}}{{R + {h_1}}}} \right) - \left( { - \frac{{GMm}}{{R + {h_2}}}} \right)\)
\( = \left( { - \frac{{GMm}}{{R + 2R}}} \right) - \left( { - \frac{{GMm}}{{R + R}}} \right)\)
\( = - \frac{{GMm}}{{3R}} + \frac{{GMm}}{{2R}}\)
\( = \frac{1}{6}\frac{{GMm}}{R}\).