A problem about "Density Matrix" (Atomic and Molecular Physics)

The wavelength of the |5S1/2, F=2> -> |5P3/2, F=3> transition of
87Rb atoms is 780.0nm and the natural linewidth of |5P3/2, F=3> states
is 6MHz. While the intensity of the radiation field is much less than
the saturation intensity, what is the on-resonance absorption cross
section of the transition? Cold 87Rb atoms uniformly form a sphere
with a radius r=1mm. A weak and on-resonance laser light passes
through the cloud and the width of the laser beam is much larger than
r. If there are 10^8 atoms in the cloud, what is the reduction of the
laser power after passing through the cloud?