Group delay related question

my prof ask me to calculate the phase of each wavelength from a relationship between group delay and wavelength:

τ = A(λ-λo)

τ: group delay
λo = 1550nm
λ: arbitrary wavelength in nm
A: coefficient in ps/nm

This formula shows us how to calculate the group delay, corresponds to a wavelength.

My task is to calculate the phase shift, corresponds to each wavelength. How can I solve this problem?

Thank you in advance,
BJ