If the change in the value of g at a height h above the surface of the earth is the same as at a depth x below it, then (both x and h being much smaller than the radius of the earth)

The value of g at the height h from the surface of earthg′=g1−2hRThe value of g at depth x below the surface of earthg′=g1−xRThese two are given equal, hence 1−2hR=1−xROn solving, we get x = 2h