Anonymous

The point of interest must be 3 times as far from the earth's center as a point on the earth's surface would be. So the height of such a point is 2r.

Anonymous

g = MmG/r^2 where g is the force of gravity, M is the mass of the Earth,m is the mass of an object and r is the distance from the centre of the Earth. so sqrt(9) = 3 radii

Anonymous

1 g = GM/r² g/9 = GM/h² GM/(9r²) = GM/h² 9r² = h² h = 3r