General

Need some help with Physics

question is:

Light of wavelength 590nm is incident normally on a diffraction grating having 750 lines per millimetre.
The diffraction grating formula may be expressed in the form
dsin(theta) = n(lameda)

Calculate the value of d, in metres for this grating

I can just find the answer in the mark scheme i have here since i'm doing a past paper, but that won't do me any good. Can someone explain why it has to be 1/(750x10^3)? how does n and (lameda) = 1?