A plane electromagnetic wave of frequency 75 Hz travels in free space along the X-direction. At a particular point in space and time $\vec{E}=9.6 \hat{j}$. What is $\vec{B}$ at that point? (E = Electric field, B = Magnetic field)

$3.2 \times 10^{-8} \hat{k} T$

Direction of propagation of EM Wave is given by 

$ \vec{S} = \vec{E} \times \vec{B}$

Direction of propagation is along X axis and Electric Field is along Y axis, Hence direction of Magnetic field($\vec{B})$ is along Z-axis.

Magnitude of B is $ B = \frac{E}{c} = \frac{9.6}{3\times 10^8} = 3.2\times 10^{-8}T$

$\vec{B} = 3.2\times 10^{-8} T\hat{k}$