The compound angle formulas can be used to derive many other formulas. One of these is the formula

a) Show that the formula is valid for x = 2pi/3 and y = pi/3. (I did this, both sides were equal to square root of 3)

b) Use appropriate equivalent trigonometric expressions to derive a similar formula for sin x - sin y.

Answer for b:

How would I show my work as to get to that answer for b? I don't really know what b is even asking.