I think I can answer this. The formula for diffraction grating is,
d sin θ = nλ
Where d = spacing of the lines, θ is the angle of the nth order from the undeviated beam, n is the order # and λ is the wavelength. Here we have to find n which is equal to 1/d,
d sin 22.86 = 2 * 485.6 * 10^-9
d = 2.5 * 10^-6
Lines per m = 1/d = 400000
Lines per mm = 400
I hope that's the right answer.