The yellow light of a sodium lamp is not a single wavelength…

The yellow light of a sodium lamp is not a single wavelength. It has two very close lines, of wavelengths $589.0$ nm and $589.6$ nm. A teacher wants to show the two lines as separate bright lines with a diffraction grating that has $600$ lines in each millimetre. She asks how good the grating must be. The lamp gives a wide parallel beam that falls on the whole grating, and she is free to use the first order or the second order of the pattern. The resolving power of a grating is $R = \lambda/\Delta\lambda$, and it equals $mN$, where $m$ is the order and $N$ is the number of lines that the light falls on. Find the resolving power that is needed to separate the lines. Find the number of lines that must be lit in the second order. Find the width of the grating that must be lit in the first order.