m<FIB = 90 - (4x+3)
m<FIB = 87 - 4x
---------------------
m<HFG = 90 - (5x + 4)
m<HFG = 86 - 5x
-------------------------------
<BCF = 180 - (<CBF + <CFB)
<CBF = 10x + 3
<CFB =90 - <CFD
but <CFD = <HFG = 86 - 5x (vertical angles)
<CFB =90 -(86 - 5x) = 4 + 5x
<BCF = 180 - (10x + 3 + 4 + 5x)
<BCF = 180 - (15x + 7)
<BCF = = 173 - 15x
