Do you mean find an exponential fit to your data? I presume your data is in the form of pairs {months since 13/31/03, number of users}? Not sure what you mean by "algebraically step by step"? Do you mean set up the calculation using the numbers given? Otherwise, Mathematica has a function called FindFit[] which you can use to fit an exponential to your data. When I fit what I understand to be your data to the exponential form, a e^{-b x}, I get a = 0.414272 and b = -0.0975048. Graphically this is a pretty good fit.
