Answer: 3
Explanation:
Triangle DCB is similar to Triangle ECA by AAA.
Since EA and DB are parallel lines, that makes Angle CDB = Angle CEA and Angle CBD = Angle CAE because they are corresponding angles.
Since the two triangles are parallel, we can set up a ratio.

Now we plug in the numbers given.

Now we can cross multiply
6CA = 72
Therefore, CA = 12.
AB = CA - CB
So AB = 12 - 9
AB = 3