Answer
$S=265 \mathrm{~W} / \mathrm{m}^2 $
Work Step by Step
From $\vec{S}=\vec{E} \times \vec{B} / \mu_0$, we find the magnitude of the Poynting vector to be
$$
S=\frac{E B}{\mu_0}\\=\frac{(0.0833 \mathrm{~V} / \mathrm{m})\left(4.0 \times 10^{-3} \mathrm{~T}\right)}{2\left(4 \pi \times 10^{-7} \mathrm{~T} \cdot \mathrm{m} / \mathrm{A}\right)}\\=265 \mathrm{~W} / \mathrm{m}^2
$$
