from what i can gather, they want the Keq for the reaction as written (i.e. forward reaction), so yes Keq = [C]^2/[A_2] as this is the actual formula for Keq.
if you started off with C, you'd essentially flip the reaction so then Keq for that reverse reaction would be the reciprocal, but only for that reverse reaction (i.e. for 2C <--> A_2 + B)
so since the reaction they give you is the forward reaction and they want the Keq for that, you'd treat C as the product since it's the product in the forward reaction.
essentially what you could do is find the Keq of the reverse reaction using C as the reactant with the info given, then take the reciprocal of that, which would give you Keq of the forward reaction as written.
hopefully this makes some sense.