Answer
$\frac{d m}{d t}=6.2 \times 10^{11} \mathrm{~kg} / \mathrm{s} $
Work Step by Step
Keeping in mind that a watt is a joule per second, the rate is
$
\frac{d m}{d t}=\frac{3.9 \times 10^{26} \mathrm{~W}}{6.3 \times 10^{14} \mathrm{~J} / \mathrm{kg}}\\=6.2 \times 10^{11} \mathrm{~kg} / \mathrm{s} .
$
