Two parallel slits are illuminated by light composed of two wavelengths, one of which is 486 nm.

Two parallel slits are illuminated by light composed of two wavelengths, one of which is 486 nm. On a viewing screen, the light whose wavelength is known produces its third dark fringe at the same place where the light whose wavelength is unknown produces its fourth-order bright fringe. The fringes are counted relative to the central or zeroth-order bright fringe. What is the unknown wavelength?