Intensity of light is $I=\frac{\mathrm{Power}}{\mathrm{Area}}=\frac{200\times {\displaystyle \frac{3.5}{100}}}{4\pi {\left(4\right)}^2}=0.034 \mathrm{W} {\mathrm{m}}^{-2}$

Using the relation between magnetic field and intensity, $I=\frac{B_0^{2}c}{2{\mu }_0}$

We have, magnetic field as $B_0=\sqrt{\frac{I\times 2{\mu }_0}{c}}=\sqrt{\frac{0.034\times 2\times 4\pi \times {10}^{-7}}{3\times {10}^8}}=1.69\times {10}^{-8} \mathrm{T}\approx 1.71\times {10}^{-8} \mathrm{T}$