Block carrying a charge on a frictionless horizontal track

[nosracsan]

Homework Statement

A 4.00 kg block carrying a charge Q=50.0 µC is connected to a spring for which k=100N/m (see diagram). The block lies on a frictionless horizontal track, and the system is immersed in a uniform electric field of magnitude E = 5.00x10^5 V/m, directed as shown. If the block is released from rest when the spring is unstretched (at x = 0), (a) by what maximum amount does the spring expand? (b) What is the equilibrium position of the block?


See attached MS Word Document for diagrams / further information. Thanks!

 

[ApexOfDE]

I think at equilibrium pos, electric force is equal to restoring force. And this pos should be a max amount that spring expands because there is no other force. :-?