Answer
$E_{rms} = 18.2~V/m$
Work Step by Step
We can find the $rms$ value of the electric component:
$E_{rms} = \frac{E}{\sqrt{2}}$
$E_{rms} = \frac{cB}{\sqrt{2}}$
$E_{rms} = \frac{(3.0\times 10^8~m/s)(85.8\times 10^{-9}~T)}{\sqrt{2}}$
$E_{rms} = 18.2~V/m$
