As seen in Fig. 13-37, two spheres of mass m and a third sphere of mass M form an equilateral triangle, and a fourth sphere of mass is at the center of the triangle. The net gravitational force on that central sphere from the three other spheres is zero. (a) What is M in terms of m? (b) If we double the value of m4, what then is the magnitude of the net gravitational force on the central sphere?