use double angle formulas

Question

find cos 2a using double angle identities given cos a=√-57/19 where pi/2<a<pi.

I drew a triangle in the second quadrant, but cos2a has three formulas how can I tell which formula is suitable for this question thanks.

Answer 1

As only given is cosβ, then use cos2β=2cos²(β) -1

Answer 2

cos a is given. Use the formula that uses only cosine.