Answer
$$4.3 \times 10^{9} \mathrm{kg} / \mathrm{s}
$$
Work Step by Step
From the Einstein relation $E=M c^{2}$ we get $P=d E / d t=c^{2} d M / d t,$ or
$$
\frac{d M}{d t}=\frac{P}{c^{2}}=\frac{3.9 \times 10^{26} \mathrm{W}}{\left|3.0 \times 10^{8} \mathrm{m} / \mathrm{s}\right|^{2}}=4.3 \times 10^{9} \mathrm{kg} / \mathrm{s}
$$
