If AC must be parallel with BD and m∠6 is doubled, m∠3 must be doubled too.
This is because of the following rule:
When two lines are parallel, which is the occasion with AB and CD, then the corners in which the intecepting parallel lines (AC and BD) stand must be the same. So corner 6 is equivalent to corner 3, corner 1 and corner 5. Corner 2 and corner 4 are equivalent too.
