imagine a sphere of radius 7x10^12m centered on the star; the surface area of this sphere is 4 pi r^2
thus, the total output of the star is distributed evenly over this sphere, so we have
15.4W/m^2 = L/(4 pi r^2) where L is the total output (luminosity)
L = 15.4 W/m^2 x 4 pi (7x10^12m)^2 = 9.48 x10^27W