What is the magnitude/direction of the E field?

[Vitani11]

Homework Statement

A bob of mass 0.005kg is attached to a pendulum and has a charge of -8.0 uC. The pendulum is 1.0m long and has a periods of 1.2 s when it's puts it in a vertical uniform electric field E. What are the magnitude and direction of E?

Homework Equations

∑F_y = qE - mg = ma

q is charge
E is electric field
m is mass
g is gravity
a is acceleration
L is length
T is period

The Attempt at a Solution

I set up a FBD and this is what I have so far.

 

[Vitani11]

At this point I'm trying to find an equation for acceleration so I can solve for E.

 

[gneill]

Show what you've done.