The second dark band in a double-slit interference pattern is I.2 cm from the central maximum of

Question:

The second dark band in a double-slit interference pattern is I.2 cm from the central maximum of the pattern. The separation between the two slits is equal to 800 wavelengths of the monochromatic light incident (perpendicularly) on the slits. What is the distance between the plane of the slits and the viewing screen?