Physics "Physical Optics" help?

Here's the question:

A mixture of yellow light (wavelength = 582 nm in vacuum) and violet light (wavelength = 404 nm in vacuum) falls perpendicularly on a film of gasoline that is floating on a puddle of water. For both wavelengths, the refractive index of gasoline is n = 1.40 and that of water is n = 1.33. What is the minimum nonzero thickness of the film in a spot that looks the color of violet and yellow because of destructive interference?

I do not get this at all. It looks really complicated and I know it's easy to solve if you set your mind to it.... but I just can't get it. Can someone describe how to start it out and tell me the answer that he or she got so I can confirm that what I did was correct?