Answer
$2\times10^{-5}m$
Work Step by Step
Assume that the plates are large enough that the electric field is uniform. Use equation 17–4b to find the separation.
$$|E|=\frac{V_{ba}}{d}$$
$$d=\frac{V_{ba}}{|E|}=\frac{45V}{3\times10^6V/m}=1.5\times10^{-5}m\approx 2\times10^{-5}m$$