Answer
$0.556 * 10 ^{10}kgs^{-1}$.
Work Step by Step
From Einstein's equation, $E = mc^{2}$, so, $m = \frac{E}{c^{2}}$
$m = \frac{5 * 10^{26}Js^{-1}}{(3*10^{8}ms^{-1})^{2}} = 0.556 * 10 ^{10}kgs^{-1}$
Hence, the mass loss of sun is $0.556 * 10 ^{10}kgs^{-1}$.
