Answer
$0.48\frac{KV}{m}$
Work Step by Step
As $E_{rms}=\sqrt{\frac{P_{avg}}{Ac\epsilon_{\circ}}}$
We plug in the known values to obtain:
$E_{rms}=\sqrt{\frac{2.8\times 10^{-3}}{\pi(1.2\times 10^{-3})(3.00\times 10^8)(8.85\times 10^{-12})}}$
$E_{rms}=0.48\frac{KV}{m}$
