The electric field part of an electromagnetic wave in vacuum is $\overrightarrow{\mathrm{E}}=3.1\frac{\mathrm{N}}{\mathrm{C}}\cos \left[\left(1.8\frac{\mathrm{rad}}{\mathrm{m}}\right)\mathrm{y}+\left(5.4\times {10}^8\frac{\mathrm{rad}}{\mathrm{s}}\right)\mathrm{t}\right]\widehat{\mathrm{i}}$. The amplitude of magnetic field part of the given wave is:

From the equation, the magnitude of electric field is ${\mathrm{E}}_0=3.1 {\mathrm{NC}}^{-1}$.

The electric field amplitude and magnetic field amplitude is given by ${\mathrm{E}}_0={\mathrm{cB}}_0$

$\Rightarrow {\mathrm{B}}_0=\frac{{\mathrm{E}}_0}{\mathrm{c}}=\frac{3.1}{3\times {10}^8}=1.03\times {10}^{-8} \mathrm{T}$