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Gravitational attraction between two objects =
   F = G M1 M2 / d²

 where d = distance between the centers of mass of the two objects
             = 2 R

     F = G M² / (2R)² = (G M²/4) R⁻²
 So the force   α  (2 R)⁻²    or  R⁻²
   x = - 2

2 3 2
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But the answer is 4