T = [66 60 55 51.5 47 43 41 39 35]; 
t = [0 5.5 12 16.5 24.5 32.5 38.5 44 61];      % time has to start with 0
n = length(T);


plot(t,T,'.');

Ts = 24; 

k = -log( (T(n) - Ts)/ (T(1) - Ts)) / t(n)                 % using the last observation to fit k


   hold on
   tc = 1:100;     % plotting the curve
   plot(tc, Ts + (T(1) - Ts)*exp(-k*tc));
     hold off
  
% what if we used t(7) to fit      
     
k = -log( (T(7) - Ts)/ (T(1) - Ts)) / t(7)                 % using the 7-th observation to fit k

   hold on
   tc = 0:100;     % plotting the curve
   plot(tc, Ts + (T(1) - Ts)*exp(-k*tc),'r');
     hold off