Answer
$E_{rms} = 258~V/m$
Work Step by Step
We can find the intensity of the laser:
$I = \frac{P}{A}$
$I = \frac{P}{\pi~r^2}$
$I = \frac{10.0\times 10^{-3}~W}{(\pi)~(4.25\times 10^{-3}~m)^2}$
$I = 176.2~W/m^2$
We can find $E_{rms}$:
$E_{rms}^2 = \frac{I}{c~\epsilon_0}$
$E_{rms} = \sqrt{\frac{I}{c~\epsilon_0}}$
$E_{rms} = \sqrt{\frac{176.2~W/m^2}{(3.0\times 10^8~m/s)~(8.85\times 10^{-12}~C^2/N~m^2)}}$
$E_{rms} = 258~V/m$
