Gauss's law, determing average volume charge density

[Breedlove]

Homework Statement

In the air over a particular region at an altitude of 500 m above the ground, the electric field is 120 N/C directed downward. At 600 m above the ground, the electric field is 100 N/C downward. What is the average volume charge density in the layer of air between these two elevations? Is it positive or negative?

Homework Equations

\Phi_net=\stackrel{Q}{\epsilon}
\rho=\stackrel{Q}{v}
\Phi=\intE(dot)dA

The Attempt at a Solution

I substituted \rhoV for Q in
\Phi_net=\stackrel{Q}{\epsilon}
and then set that equal to EA from \Phi=\intE(dot)dA, and solved for \rho, getting \rho=E/(h\epsilon)
h is coming from A/V

Am I going about this the right way? My next step would be to find \rho at both elevations and average them. Are the steps that I have taken this far correct?

Thanks for whatever help you can offer!

Oh, and anything that looks like a superscript or subscript is just an error on my part, I didn't mean for the equations to look like they had them. I'm still pretty new with it.
Thanks!

 

[Vuldoraq]

Looks like a good way to go about the calculation. As a safety check make sure your units on both sides of your equation are consistent, that way you know when your definitely on the wrong track! (P.S. I think your units are good in this case).