22-15

$${E = \int dE\, \cos \theta = \frac{z \lambda}{4 \pi \varepsilon_0 \left( z^2 + R^2 \right)^{3/2}} \int_0^{2 \pi R} ds = \frac{z \lambda (2 \pi R)}{4 \pi \varepsilon_0 \left( z^2 + R^2 \right)^{3/2}}}$$