where $E_{500}=120\U{N/C}$ and $E_{600}=100\U{N/C}$. From Gauss's
law,
\begin{align}
- \Phi_E &= \frac{q_\text{in}}{\varepsilon_0} = (E_{500}-E{600})A \\
- q_\text{in} &= (E_{500}-E{600})A\varepsilon_0 \;.
+ \Phi_E &= \frac{q_\text{in}}{\varepsilon_0} = (E_{500}-E_{600})A \\
+ q_\text{in} &= (E_{500}-E_{600})A\varepsilon_0 \;.
\end{align}
This gives an average volume charge density of
\begin{equation}
\rho \equiv \frac{q_\text{in}}{V}
- = \frac{(E_{500}-E{600})A\varepsilon_0}{A(h_{600}-h_{500})}
- = \frac{E_{500}-E{600}}{h_{600}-h_{500}}\varepsilon_0
+ = \frac{(E_{500}-E_{600})A\varepsilon_0}{A(h_{600}-h_{500})}
+ = \frac{E_{500}-E_{600}}{h_{600}-h_{500}}\varepsilon_0
= \frac{20\U{N/C}}{100\U{m}}\cdot8.85\E{-12}\U{C$^2$/N$\cdot$m$^2$}
= \ans{1.77\E{-12}\U{C/m$^3$}} \;.
\end{equation}