(a) Non-relativistically,  z = v/c = (47500 km/s) / (300000 km/s) = 0.158

    Relativistically,  z = sqrt( (1 + v/c) / (1 - v/c) ) -1
    So z = sqrt( 1.158 / 0.842 ) - 1 = 0.173

    There is somewhat of a difference, about 10% or so.  This level of inaccuracy
    is about the maximum tolerable error for this kind of measurement.


(b) v = Hr and v = 47500 km/s, and H = 70 km/s/Mpc, so
       r = (47500 km/s) / (70 km/s/Mpc) = 680 Mpc

(c) F = L/(4 * pi * r^2), so L = F * 4 * pi * r^2.  F = 3 * 10^-13 W/m^2, so
    L = 3 *  10^-13 W/m^2 * 4 * 3.14 * (680 Mpc * 3.1 * 10^22 m/Mpc)^2
      = 1.7 * 10^39 Watts

The Sun's luminosity is 3.9 * 10^26 Watts.  A Watt is 1 Joule/sec,
so 3C 273 produces 1.7 * 10^39 Joules in one sec.  That means the Sun 
has to shine, to equal that amount of energy,

 1.7 * 10^39 J
--------------- = 4.4 * 10^12 s; at 3.15 * 10^7 seconds per year, that's
3.9 * 10^26 J/s

 (4.4 * 10^12) / (3.15 * 10^7) = 140,000 years.