Solution :
$B= \large\frac{E}{C}$
$\quad= \large\frac{6.3 v/m}{3 \times 10^8m/s}$
$\quad= 2.1 \times 10^{-8}\;T$
As $\overrightarrow{E}$ is along y- direction and wave is travelling along x- direction .
$\overrightarrow{B}$ is along z direction
$\qquad= 2.1 \times 10^{-8} \hat k \;t$