(a) F = L/(4 pi r^2);
                            L
     0.002 W/m^2 = --------------------- ; so
                   4 * 3.14 * (10^6 m)^2


     0.002 W/m^2 * 4 * 3.14 * (10^6 m)^2 = L = 2.5 * 10^10 W.


(b) F = L/(4 pi r^2);
                      2000 W                         2000 W
     0.015 W/m^2 = -------------- ; so r^2 = ----------------------
                   4 * 3.14 * r^2            4 * 3.14 * 0.015 W/m^2

     r = sqrt(10615 m^2) = 103 m.