Answer
$E = 2.9\times 10^7~N/C$
Work Step by Step
We can use Equation (22-26) to find the magnitude of the electric field:
$E = \frac{\sigma}{2\epsilon_0}(1 - \frac{z}{\sqrt{z^2+R^2}})$
$E = \frac{5.3\times 10^{-6}~C/m^2}{(2)(8.854\times 10^{-12}~C^2/N~m^2)}(1 - \frac{0.12~m}{\sqrt{(0.12~m)^2+(0.025~m)^2}})$
$E = 2.9\times 10^7~N/C$
