Solving a problem in physics section 6 point 78 optics

78. The light normally incident on the grating width l = 6,5 cm, having 200 grooves per millimeter. The test spectrum contains spectral line with λ = 670,8 nm, which consists of two components, differing by δλ = 0,015 nm. Find: a) the order in which these spectral components will be allowed; b) the smallest difference in wavelength, which can solve this grid in λ ≈670 nm.