The magnitude of the magnetic field will be given by the magnitude of the electric field divided by the speed of light. From the direction of the electromagnetic wave and the electric field we can find the direction of the magnetic field as they are mutually perpendicular to each other.

The magnitude of B is 

B = CE

= 6.3 /3×108

= 2.1×10−8T

To find the direction, we note that E is along y – direction and the wave propagates along x – axis

Therefore, B should be in a direction perpendicular to both x – and y – axes.

Using vector algebra, E × B should be along x – direction.

since, ( + j ) ×(+k)=i,

so, B is along + k or the z – direction

so B = 2.1 × 10−8k