segment addition postulate

how to find the length of a line segment given a congruent line segments length and the sum of both.
here is line ac. b is its midpoint. [br]|--------------------|--------------------|[br]a b c[br]ab = 12 - (4/2) we know that ab + bc = ac (segment addition postulate).[br]ac = 20 so 12 - (4/2) + x = 20.[br]find bc. solve the algebraic equation. side note: we also know [br]                  that ab = bc because they[br] are congruent sides of a [br]                    midpoint.

Information: segment addition postulate