please help

Angle opposite 57    is   180 - 57 = 123^o

 

ENTIRE lower arc from 57  to B is then   2 x 123 = 246^o

Arc encompassed by angle next to 'b' is thus   246 - 100 = 146^o

    meaning the supplementary angle with b   is     73^o  

                      b + 73 = 180       b = 107^o