[iklerini (OP)]

I am given that the grating has 500 lines per mm and it is illuminated with a light of wavelength 510nm. The question is asking for haw many diffraction orders are seen and what the angle is of each. The only equations I know have both the angle and number of orders in the problem so I cant figure it out with having 2 variables in the problem.

[Colin2B]

Firstly, remember that the integer for the individual orders is one you supply.
Secondly, the orders are projected onto a screen so there cannot be any with an angle >90°
The rest is playing around with the formula. Enjoy