Answer
$8.85\times 10^{-6}C/m^2$
Work Step by Step
We can find the change in the charge density as follows:
$\Delta E=\frac{\Delta \sigma}{\epsilon_{\circ}}$
This can be rearranged as:
$\Delta \sigma=\epsilon_{\circ}\Delta E$
We plug in the known values to obtain:
$\Delta \sigma=(8.85\times 10^{-12})[7.0\times 10^5-(-3.0\times 10^5)]$
$\Delta \sigma=8.85\times 10^{-6}C/m^2$
