How Do You Calculate the Mass of a Particle in an Electric Field?

[krony23]

1. Homework Statement
In a laboratory, a particle of charge -6C is held stationary because it is placed in an electric field E=(0,0,-15)N/C which suspends it against gravity. What is the mass of the particle? Give answer in kg.

2. Homework Equations
I THINK
E= F/q
F= ma or F= G m1 m2 / r^2


3. The Attempt at a Solution
Ok, so I haven't actually had a chance to attempt this yet because A) I don't understand the notation of E in this equation, so I don't know if it is acceptable to just use -15 for E, and B) if I even have the right equations to use, I don't know what the acceleration is or how to use the second equation. Please help

 

[kuruman]

The problem states "... which suspends it against gravity." This mens that the particle is not moving. What is its acceleration then? How many forces act on the particle, in what directions do they point and how are their magnitudes related?

Note: Use mg for the force of gravity.