In triangle ABC, sin (A + B) = 3/5.
What is the value of sin C?
You don't need them.
What does $\displaystyle \sin{180^\circ}$ equal?
Once you've found that, what does $\displaystyle \sin{180^\circ}\cos{(A + B)^\circ}$ equal?
What does $\displaystyle \cos{180^\circ}$ equal?
Since you're GIVEN the value of $\displaystyle \sin{(A + B)^\circ}$, what is $\displaystyle \cos{180^\circ}\sin{(A + B)^\circ}$?