1) Have you recently studied the Law of Signs and Law of Cosines? These could be very useful, here.
2) Diameter = 2*pi*r
8.4 cm / Diameter = How many degrees of the circle?
You should have a theorem about the measure of inscribed angles. Something about "half" comes to mind.
Let's see what you get.
*EDIT: Are you sure that said "central" angle the first time? Sleep reading again, I guess.