Difference in potential energy is given by: PE=-GMm\\left(\\dfrac {1}{R_1}-\\dfrac {1}{R_2}\\right) But, R_1 = 6400 km and R_2 = 12800 km \\Rightarrow PE=-6.67\ imes10^{-11}\\left(5.97\ imes10^{24}\\right) (10) \\left(\\dfrac {1}{6400}-\\dfrac {1}{12800}\\right)\ imes10^{-3} \\Rightarrow PE = 0.00311\ imes10^{11}J