\Phi_{E1} &= \frac{-2Q + Q}{\varepsilon_0}
= \ans{\frac{-Q}{\varepsilon_0}} \\
\Phi_{E2} &= \frac{Q - Q}{\varepsilon_0} = \ans{0} \\
- \Phi_{E1} &= \frac{-2Q + Q - Q}{\varepsilon_0}
+ \Phi_{E3} &= \frac{-2Q + Q - Q}{\varepsilon_0}
= \ans{\frac{-2Q}{\varepsilon_0}} \\
- \Phi_{E1} &= \frac{0}{\varepsilon_0} = \ans{0} \;.
+ \Phi_{E4} &= \frac{0}{\varepsilon_0} = \ans{0} \;.
\end{align}
\end{solution}