85 \times 10^{-12} C^2/Nm^2$). The surface charge density is given by $\sigma = \frac{Q}{A}$, where $Q$ is the charge and $A$ is the area. In this case, the area is $A = (2 cm)^2 = (0.02 m)^2 = 4 \times 10^{-4} m^2$.
For the first electrode (at x=0), $Q_1 = -30 nC
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