Light with wavelength 648nm in air is incident perpendicularly from air on a film 8.76m thick and

Light with wavelength 648nm in air is incident perpendicularly from air on a film 8.76µm thick and with refractive index 1.35. Part of the light is reflected from the first surface of the film, and part enters the film and is reflected back at the second surface, where the film is again in contact with air
(a) How many waves are contained along the path of this second part of the light in its round trip through the film?
(b) What is the phase difference between these two parts of the light as they leave the film?