In the figure below, a central particle of mass M is surrounded by a square array of particles, separated by either distance d or distance d/2 along the perimeter of the square. The mass m is equal to 9M. What are the magnitude and direction of the net gravitational force on the central particle due to the other particles? (Use the following as necessary: d, M and G, the gravitational constant.)

 

2 4 M 7M 5M m 5M 7M M 2M