1) Given
2) Adding segments AB and BC together gives AC (Addition postulate, not too sure about the terminology)
3) Segment Addition postulate
4) Segment Addition postulate(Knowing that AC=BD, the distance between BC and CD add up to BD, shown in #3. AB and BC add together to give AC)
5) Subtraction property of Equality
Hope I helped :)
Answer is -6(see photo for step by step):
