http://session.masteringphysics.com/problemAsset/1125570/2/Walker4e.ch12.Pr042.jpg use this image for help a. Consider the four masses shown in the figure . Find the total gravitational potential energy of this system. b. How does your answer to part A change if all the masses in the system are doubled? c. How does your answer to part A change if, instead, all the sides of the rectangle are halved in length? I'm absolutely lost in this problem. I know that U=-GmMe/r where Me is the mass of earth. r= radius However, I don't know how to apply that to so many different masses. Please help and explain?!