I've made some assumptions here (as I said before, I don't know what all the letters stand for in your equation, but I had a guess). So in Absolute Temperature
-48 Deg C = 225 deg K (273-48)
Ts = 225/1 + 0.2 x 0.85 x (0.7)²
Ts = 225/1 + 0.2 x 0.85 x 0.49
Ts = 225/1.0833
Ts = 207.7 deg K
Now convert to Deg C:
207.7 deg K - 273 =
-65.3 deg C
Old Smokey seems to know what's going on, so hopefully will be able to confirm this is how it is calculated. Otherwise, maybe tell me what I've assumed incorrectly!
Edited later: I've since found out what the letters all mean...calculation above is OK I think.