Space between slits of diffraction grating? [closed]

White light enters a diffraction grating placed in front of a very wide viewing screen. Only the central white spot and the first order spectrum are visible on the screen.

How many lines per centimeter must the diffraction grating have?

I tried using the equation $d\sin{\theta} = m\lambda$, but I don't know what value I would use for $\lambda$.

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 use it to be in between visible spectrum. e.g. 5000 A – Samir Chauhan Feb 15 at 6:25

closed as too localized by John Rennie, Ϛѓăʑɏ βµԂԃϔ, Qmechanic♦Feb 15 at 7:25

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Look at the two extreme values of this spectrum, consider how large the angle $\theta$ can be at most and keep in mind that you see the (entire) first order spectrum.