Youre designing a spectrometer whose specifications call for a minimum of 5 separation between the red hydrogen- line at 656 nm and the yellow sodium

You’re designing a spectrometer whose specifications call for a minimum of 5° separation between the red hydrogen-α line at 656 nm and the yellow sodium line at 589 nm when the two are observed in third order with a grating spectrometer. Available gratings have 2500 lines/cm, 3500 lines/cm, or 4500 lines/cm. What’s the coarsest grating you can use?