All electromagnetic wave is transporting energy in the negative z...

In an electromagnetic wave, the direction of energy flow is along the direction of the wave propagation. Therefore, the wave propagates along negative z direction.

Electric field is along the positive y direction and the wave propagates along negative z direction. Thus, magnetic field is in direction perpendicular to both z and y axes.

Using vector algebra,

( - \overset{^}{k} ) = ( + \overset{^}{j} ) \times \left(\overset{^}{i}\right)

Hence, magnetic field is in positive $x$ direction.