as m abe = 2b, it seems like abe is the whole angle and abf and ebf are the parts of angle abe, then
m abe = m abf + m ebf
and we have to find m ebf so,
m ebf = m abe - m abf
now putting the values of angles abe and abf, we get
m ebf = 2b - (7b -24)
m ebf = 2b - 7b + 24
mebf = -5b + 24
