The Brainliest Answer!
Draw the figure. points B and C must both be on the circle. Now connect both points B and C to the circle center O. 
Since both AB and AC are tangent to the circle then AB = AC 
Now draw the line AO 
<OAB and <OAC are equal. 
Since BAC = 120 
<OAB = 60 and <OBA = 90 deg and therefore <AOB is 30 deg. 
So sin<AOB = sin 30 = 1/2 = AB/ OA and 
OA = 2 AB

figure and proof is also given in below attached pic
2 5 2