You can expand the RHS using compound angle formula.
Thank you for your help.
After I plugged that into the equation again the (cos a+b * cos a-b) cancels out and we are left with (-sin a + b sin a - b) - (sin a + b sin a - b) which should equal to -2 sin a + b sin a - b.
Now what would we do next? And what about the 2 in the denominator of the right hand side of the equation?