Here, `r = 3.8 xx 10^(8) m, M_(e) = 6 xx 10^(24) kg, M_(m) = 7.4 xx 10^(22) kg`
The gravitational potential energy of earth-moon system is
`U = (-GM_(e)M_(m))/(r ) = (-(6.67xx 10^(-11))xx (6 xx 10^(24)) xx (7.4 xx 10^(22)))/(3.8 xx 10^(8)) = - 7.78 xx 10^(28) J`