The two spheres in the figure are charged. Each has a mass of 0.91 kg. The lower mass is rigidly attached to the floor, with a charge of -6.9 x 10-5 C. The mass above it is held to the ceiling with a string, whose tension is 0.1 N. The masses are 1.4 m apart. What is the charge on the upper sphere (and make sure you get the sign right)?