Adding/subtracting sinusoids
The easiest way (to my mind) to solve the problem is to


Use the identity $\sin(A\pm B) = \sin A \cos B \pm \cos A \sin B$, substituting the known numerical values of $\cos B$ and $\sin B$,
Gathering the results to express your sum of sinusoids in the form of $C \sin A + D \cos A$,
Expressing the resulting function as $\sqrt{C^2+D^2} \sin\left(\omega t + \theta\right)$