Finding charge Q from kinetic energy using electric potential energy

[melst]

Homework Statement

A point charge Q resides at the origin. A particle of mass 0.001kg carries a charge of 6 microC. The particle is released from rest at x = 3m. Its kinetic energy as it passes x = 0.5m is 0.4J. The coulomb constant is 8.99x10^9Nm^2/C^2.
Find the charge Q. Answer in microC.

Homework Equations

I use kqQd = -k

The Attempt at a Solution

(8.99x10^9Nm^2/C^2)(6 microC)Q[(1/0.5)-(1/3]) = -0.4
Q=-4.44x10^-12

I don't know what I did wrong and is kinda confused. Can anyone help?

 

[Jebus_Chris]

Use conservation of energy.
$$U = k\frac{q_1q_2}{r}$$

 

[melst]

so is the mass not important??

 

[Jebus_Chris]

Mass is important for the kinetic energy calculation. You never have to use mass though since they give you the kinetic energy. Just write an equation for conservation of energy. You start with all potential and then some of the potential energy turns into kinetic energy.