Hello to whoever is reading this....I got a little stuck on this problem out of my AP physics homework. If you could help me out a bit I would really appreciate it. I've tried it a couple times and I feel as if I'm getting the right answer, but it still remains incorrect. The problem is below, please if you do know how to solve it, don't just tell me the answer, show me how you got it so I can use the example for future problems. Thanks so much 
Two spheres are mounted on identical horizontal springs and rest on a frictionless table. When the spheres are uncharged, the spacing between them is dx = 0.048 m, and the springs are unstrained. When each sphere has a charge of +2.8 µC, the spacing doubles. Assuming that the spheres have a negligible diameter, determine the spring constant of the springs. (The distance between the uncharged spheres is dx = 0.048.)
Answer: K= _______N/m
Two spheres are mounted on identical horizontal springs and rest on a frictionless table. When the spheres are uncharged, the spacing between them is dx = 0.048 m, and the springs are unstrained. When each sphere has a charge of +2.8 µC, the spacing doubles. Assuming that the spheres have a negligible diameter, determine the spring constant of the springs. (The distance between the uncharged spheres is dx = 0.048.)
Answer: K= _______N/m