Solving a problem in physics section 35 point 2 optics

2. The diffraction grating length l = 5 mm may allow spectrum two first order spectral line of sodium (λ1 = 589,0 nm and λ2 = 589,6 nm). Determine the angle at which the spectrum of the third order will be observed with light λ3 = 600 nm incident on the grating normal.