A film of oil of index of refraction n 143 floats on water

Question

Question: A film of oil of index of refraction n = 1.43 floats on water (n = 1.33). When illuminated with white light at normal incidence, light of wavelengths 686 and 490 nm is predominant in the reflected light.

Part A: Determine the minimum possible thickness of the oil film.

..... nm

I want help to find the minimum possible thickness of the oil film.