/sci/ - Science & Math

Hello my fellow /sci/ geeks, I really need help with this question in for my homework! Please help out :) (NOTE: Even my techer can't figure it out, so if you solve it, you are better than a higschool senior teacher at physics)

"The earth's gravityforce on a marble with the mass of 1,0kg is 9,8N on the earth's surface. Let us imagine that we place the marble outside of the earth at a distance of twice the radius of the earth from the earth's center point."
Question: "At a certain distance the force is 4,9N. What is the distance?"

(Don't forget a calculation/explanation of how you solved it)
I appreciate all the answers that you come with! Don't let me down!