In an electromagnetic wave in free space the root mean square value...

Speed of electromagnetic wave $c$ is given by, $\dfrac{{E}_{rms}}{{B}_{rms}} = c$
${B}_{rms} = \dfrac{{E}_{rms}}{c}$

$= \dfrac{6}{3 \times {10}^{8}}$

${B}_{rms} = 2 \times {10}^{- 8}$

${B}_{rms} = \dfrac{{B}_{0}}{\sqrt{2}}$

${B}_{0} = \sqrt{2} \times {B}_{rms}$

$= \sqrt{2} \times 2 \times {10}^{- 8}$

$= 2.83 \times {10}^{- 8}T$