A He Ne gas laser which produces monochromatic light of a know wavelength = 6.328 x 10-7m

Question:

A He∙ Ne gas laser which produces monochromatic light of a know wavelength λ = 6.328 x 10-7m is used to calibrate a reflection grating in a spectroscope. The first-order diffraction line is found at an angle of 21.5o to the incident beam. How many lines per meter are there on the grating?