The electric field of a plane wave in vacuum is Ex = 120 sin(kz - ω t) in V/m…

Step 1. The intensity. $I = \tfrac{1}{2}\varepsilon_0 cE_0^2 = \tfrac{1}{2} \times 8.85 \times 10^{-12} \times 3.00 \times 10^{8} \times (120\ \text{V/m})^2 = 19.1\ \text{W/m}^2$.

Step 2. The rms electric field. $E_{rms} = \dfrac{E_0}{\sqrt{2}} = \dfrac{120\ \text{V/m}}{\sqrt{2}} = 84.9\ \text{V/m}$.

Step 3. The rms magnetic field. $B_{rms} = \dfrac{E_{rms}}{c} = \dfrac{84.9\ \text{V/m}}{3.00 \times 10^{8}\ \text{m/s}} = 283\ \text{nT}$.