Electrostatics

$\frac{-9 q^2}{4 \pi \varepsilon_0 d}$

$\frac{-7 q^2}{4 \pi \varepsilon_0 d}$

$\frac{-q^2}{4 \pi \varepsilon_0 d}[9-2 \sqrt{2}]$

$\frac{-q^2}{4 \pi \varepsilon_0 d}[9+2 \sqrt{2}]$