Two parallel slits are illuminated by light composed of two wavelengths. One wavelength is A = 645

Two parallel slits are illuminated by light composed of two wavelengths. One wavelength is λA = 645 nm. The other wavelength is λB and is unknown. On a viewing screen, the light with wavelength λA = 645 nm produces its third-order bright fringe at the same place where the light with wavelength λB produces its fourth dark fringe. The fringes are counted relative to the central or zeroth-order bright fringe. What is the unknown wavelength?