Simple harmonic motion...mass attached to spring with charge and in an E field

dswatson

A block having mass m and charge +Q is connected to a spring having constant k. The block lies on a frictionless horizontal track, and the system is immersed in a uniform electric field of magnitude E, directed as shown in the figure below. The block is released from rest when the spring is unstretched (at x = 0).

http://www.webassign.net/serpop/p20-6.gif

(c) Show that the block's motion is simple harmonic and determine its period. (Use Q, E, m, and k as necessary.)

I did a,b, and d without a problem. I am not sure of the equations to use for simple harmonic motion. If someone could help walk me through this I would greatly appreciate it. Thank you in advance