Wavelength of a light whose second-order dark band forms a diffraction angle of 15.0° when it passes through a diffraction grating that has 250.0 lines per mm

The formula needed to solve this is (assuming there is only 1 slit, n = 1)
λ = d sin  θ
where d is the diffraction grating
θ is the diffraction angle
substituting the values,
λ = 250 sin 15
λ =   64.70 mm

Indoraptor for sure