Solving a problem in physics section 2 point 61 optics

61. The narrow beam of X-radiation is incident at a glancing angle θ = 60 ° to the natural facet monocrystal NaCl (M = 58.5 · 10-3 kg / mol), a density ρ = 2,16 g / cm3. Determine the wavelength λ of the radiation, if a mirror image of this face there is a maximum of third order.