Please give me a hint on how to solve this. I have no idea how to start.
Two parallel light rays of wavelength λ originate from the same light source. During
Their journey, one of them had to go through a transparent slab of thickness l and
Refractive index n. Find their phase difference.

Consider that the wavelength of the light whilst in the material is equal to \lambda/n. Hence the number of cycles that the light wave experiences while traveling through the material is \frac I {\lambda /n} . Meanwhile the light ray that does not go through the material experiences I/ \lambda cycles. The difference represents the phase shift, in cycles - be sure to convert to radians.

Understood. Thanks.
I have now prepared a complete working of this based on this answer and my understanding.