22-7

$${E = \frac{q}{4 \pi \varepsilon_0 z^2} \frac{2 d/z}{\left( 1 - \left( \frac{d}{2z} \right) \right)^2} = \frac{q}{2 \pi \varepsilon_0 z^3} \frac{d}{\left( 1 - \left( \frac{d}{2z} \right) \right)^2}}$$