Answer:
At the earth's surface g = G M / R^2 where G is the gravitational constant
at H g1 = G M / (R + H)^2 using Gauss' theorem for enclosed mass
g1 = G M / (R^2 + 2 R H) ignoring H^2 as it is small compared to R^2
g / g1 = (R^2 + 2 R H) / R^2 = 1 + 2 R H
g = g1 + 2 R H g1
g1 - g = - 2 R H or H = (g1 - g) / 2 R