yes, so note that AOB and BOC add up to AOC.
AOB + BOC = AOC
sub in the values given in the question to obtain
3x + 4 + 8x - 28 = 108
simple one-variable equation, x = 12.
and AOB is 3x + 4, so that's 3(12) + 4 = 40 is your answer
hey, that makes cents