Interference and diffraction

A single, monochromatic indigo light source is shined through an etched, flat prism with a slit separation of .0250mm.
The resulting interference pattern is viewed on a screen 1.25m away. The third maximum is found to be 6.6cm from the central maximum. What is the wavelength of the indigo light?

You can also get the answer to this paper securely, anonymously. The paper shall be original with an accompanying originality report.