Subject: How exactly does the equation n wavelength = d sin angle work?

This is in relation to spectra of mercury. If you have a grating of 400 lines 
per mm will d equal 6x10E-6? I am asking because i have been working on physics 
coursework all day and i just cannot get this formula to work. I have tried 
using a graph to obtain the answer, because the gradient is equal to the 
wavelenght of the colour of light, but that did not help! Please help me 
understand this equation in the hope that i can then complete my coursework. 
Also can only two colours appear in 1 order when the rest have 4? 

Re: How exactly does the equation n wavelength = d sin angle work?