Solving a problem in physics section 2 point 49 optics

49. In the diffraction grating comprising n = 100 lines per mm to 1 mm, normally incident monochromatic light. Telescope spectrometer hovering over a maximum of third order. To point the telescope at another maximum of the same order, it is necessary to turn to Δφ = 20°. Determine a wavelength λ of light (Fig. A).