Chapter 7 - Trigonometric Identities and Equations - 7.3 Sum and Difference Identities - 7.3 Exercises - Page 679: 39

$20^0$

$\cos \alpha =\sin \theta \Rightarrow \alpha +\theta =\dfrac {\pi }{2}\Rightarrow \sin \left( 3\theta -15\right) =\cos \left( \theta +25\right) \Rightarrow 3\theta -15+\theta +25=\dfrac {\pi }{2}=90\Rightarrow 4\theta +10=90\Rightarrow \theta =20^0$