An isotropic point source emits light with wavelength = 589 nm. The radiation power of the source

Question:

An isotropic point source emits light with wavelength λ = 589 nm. The radiation power of the source is P = 10 W. Find:
(a) The mean density of the flow of photons at a distance r = 2.0 m from the source;
(b) The distance between the source and the point at which the mean concentration of photons is equal to r =100 cm–3.