Diffraction Gratings and Incident Light

[Dgray101]

Homework Statement

If light incident on a diffraction grating makes an angle η with respect to the normal of the grating, show that mλ=dsinθ becomes d( sin(θ-η) + sin(η) ) = mλ

Homework Equations

The Attempt at a Solution

I haven't done any work with diffraction gratings in a long time so this has thrown me for a loop :/ I can't really start the problem because I don't quite understand where to begin

 

[cheah10]

This question just simply needs the knowledge of double angle formula in trigonometry. You just need to change from sin A to sin(A-n) + sin(n).
So, what's the double angle formula for sin A?