Average power per unit area of an electromagnetic wave is given by $I=\frac{c{B}_{max}^2}{2{\mu }_0}$, here, $c$ is speed of light in vacuum and ${\mu }_0$ is permeability of free space.

Now, the amplitude of the oscillating magnetic field is $B_{max}=\sqrt{\frac{2{\mu }_{0}I}{c}}=\sqrt{\frac{2\times 4\pi \times {10}^{-7}\times 9240}{3\times {10}^8}}=8.79\times {10}^{-6} \mathrm{T}=8.8 \mathrm{\mu T}$