Inscribed angles on the same intersecting arc are all equal.
Therefore we have the equation:
[tex]3x = x+8\qquad|-x\\\\2x=8\qquad|:2\\\\\boxed{x=4}\\\\m\angle QRS=3x\to m\angle QRS=(3\cdot4)^o=12^o\\\\m\angle STU=x+8\to m\angle STU=(4+8)^o=12^o[/tex]
Answer: C.
