The H line in hydrogen has a wavelength of 656.20 nm. This line differs in wavelength from

The Hα line in hydrogen has a wavelength of 656.20 nm. This line differs in wavelength from the corresponding spectral line in deuterium (the heavy stable isotope of hydrogen) by 0.18 nm.
(a) Determine the minimum number of lines a grating must have to resolve these two wavelengths in the first order.
(b) Repeat part (a) for the second order.