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