Hi - out of a text the following example is given:

if sin2x = -cos(-x + 9) then find x. (in degrees)

1. sin2x = -cos(-x + 9) = cos(-x +9 +180)

2. but 2x - x + 9 + 180 = 90

3. x = -99

steps 1 and 3 are clear to me but I'm missing why in step 3 we can add the angles to 90. All insight appreciated. Thanks.

Chris