= Solution
Here $E_y^2+E_z^2=9A^2$, so the tip of the <electric field> traces a circle at constant magnitude: this is <circular polarization>. At fixed $x=0$, it starts at $+y$ when $t=0$ and initially rotates toward $+z$. This explicit rotation specifies the sense without relying on competing handedness conventions.
Using $\mathbf B=v_p^{-1}\hat{\mathbf x}\times\mathbf E$, with $v_p=\omega/k$, gives
$$
\boxed{\mathbf B=\frac{3A}{v_p}\bigl(\hat{\mathbf y}\sin\psi+\hat{\mathbf z}\cos\psi\bigr),\qquad B_x=0.}
$$
In vacuum replace $v_p$ by $c$. The <magnetic flux density> also has constant magnitude and is perpendicular to $\mathbf E$ at every instant.
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