how can i do this, applying Rouche Theorem and Cauchy Argument Principle?
Use Cauchy's Argument Principle.
1) where is the number of poles and is the number of zeros (both counting multiplicity). Thus, this equation tells us that but because ia analytic on . Thus, the function has two zeros and within the unit disk (and I let so if it is the same zero of multiplicity two).
2) . Thus, .
Now solve for .