Well, first of all, I've got a strong hunch that the 'sin' is the
sine of the angle (ω t + π/3) ... for a lot of reasons too numerous
to elaborate on here. That angle (the 'argument' of the sin) is the
"phase" that it's asking for.
The hunch is so strong in me that I'm going to go ahead and
assume it, and answer the question that way. So, now look at
how easy that assumption made the problem !
The phase at t=8 seconds is
(ω t + π/3) = (8 π + π/3) = (25/3)π radians
