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Sin(a)= 
To find sin(2a) = 2*sin(a)*cos(a), we ned to find cos(a)
To find sin(2a) = 2*sin(a)*cos(a), we ned to find cos(a)
We know that,
sin²(a) + cos²(a) = 1
cos(a) =
cos(a) =
Thus, cos(a) = +-
We know a is in second quadrant and thus cos(a) is - because cosine is always negative in the second quadrant.
Therefore,
sin(2a) = 2*sin(a)*cos(a) =