[High School Physics] Mass of a charge within an electric field

I am stumped with this one. There is a charge hanging from a string (of an unknown length) in the centre of two parallel plates that creates an electric field of 8x10^5N/C. The charge hanging has a value of 3x10^-5C and because of the electric field the charge is 5 degrees off vertical. Given this information, determine the mass of the charge.

I can't figure out where to start, I was thinking originally E = F/q and F = ma then F =ma and F= Eq there for Eq = ma, rearrange for m and get Eq/a but obviously I have no acceleration. I just don't know where to go with this.

EDIT:

Here is a picture of the situation with know values, also keep in mind the top of the string is at the centre between the two parallel plates.