Two charged spheres are connected to a spring as shown. The unstretched length of the spring is 14cm.

a) With Qa=6 uC and Qb=-7 uC, the spring compresses to an equilibrium length of 10 cm. Calculate the spring constant. b) Qb is now replaces with a different charge Qc. The spring now has equlibirum length of 20 cm. What is the magnitude of the charge Qc? c) What is the sign of Qc? How do you know this?