A totally reflecting disk has radius 9.00 micrometers, thickness 2.00 micrometers, and average density 700 kg/m^3. A laser has an average power output Pav spread uniformly over a cylindrical beam of radius 2.00 mm. When the laser beam shines upward on the disk in a direction perpendicular to its flat surface, the radiation pressure produces a force equal to the weight of the disk. What value of Pav is required?
(The answer is 25.9 W, but I'd like to know how to get it)

What average laser power is required if the radius of the disk is doubled?