Maybe I am mistaken, but here's my logic: a=Fnet/m in which Fnet is m*g-Cd*v then differentiate to go to (in TI-89 speak) int(1/(g-Cd*v/m),0,v)=t then, use u-substitution where u=g-Cd*v/m ending up that dv=-m/Cd*du and that (-m/Cd*ln(abs(g-Cd*v/m),0,v)=t then solve for v by doing e^... and you get v...