We know that the ratio of the magnitudes of electric and magnetic fields equals the speed of light in free space i.e., $c=\frac{E_0}{B_0}$. And in an electromagnetic wave, the electric field and magnetic field are perpendicular to one another.

So, using the relation, the magnetic field along $y‐$axis is $B_0=\frac{E_0}{c}=\frac{8.7}{3\times {10}^8}=2.9\times {10}^{-8} \mathrm{T}$