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Note that COB  + AOB  =  180   ......so....

COB + 87   = 180      subtract 87 from both sides

COB  =  93

And BAC is an inscribed angle that intercepts the same arc as central angle COB.....but.....an inscribed angle  intercepting the same arc as a central angle measures 1/2 of that central angle...

So.....BAC   = (1/2)COB   =  (1/2)93  =  46.5