The intensity of the sunlight that reaches Earth's upper atmosphere

The intensity of the sunlight that reaches Earth’s upper atmosphere is approximately 1400 W/m2W/m2.

(a) What is the total average power output of the Sun, assuming it to be an isotropic source?
(b) What is the intensity of sunlight incident on Mercury, which is 5.8×10105.8×1010 m from the Sun?