Evaluate parameters of an ODE

Dear all I have an ODE like this: dXdt = R1*R2*P + C*dPdt + R1/R2 * X. Say I couldnt find an analytic solution for this ODE, and hence I can only solve it with a solver in matlab. Now, if I have measured data of P and X (say data is stored in two arreys called P_data and X_data), but I want to evaluate R1, R2 and C which can produce the data stored in X_data given P_data.

BTW, R1, R2 and C are single values.

I know that I can use fminsearch or lsqcurvefit to perform a similar task if the function of X is an algebraic equation instead of an ODE. But now I have an ODE, so dont know how to solve the problem.

Please help, many thanks!