The two spheres in the figure are charged. Each has a mass of 0.30 kg. The lower mass is rigidly attached to the floor, with a charge of -2.1 x 10-5 C. The mass above it is held to the ceiling with a string, whose tension is 1.8 N. The masses are 0.1 m apart. What is the charge on the upper sphere (and make sure you get the sign right)?