Answer:
3a
Step-by-step explanation:
As the figure is a square, any given side will be parallel to the one that is opposite to it. Side CB is parallel to side AD, which extends from the origin point (0,0) to 3a. Given this fact, we can assume that AD has a length of 3a, and since all sides of a square are of the same length, we can assume the same length for side CB.
