From the image, it appears that the vectors AB and CD are parallel and have the same direction. Therefore, if AB is represented by the vector a, then CD can also be represented by the same vector a. So, CD = a. This is the simplest form of the expression. Please note that this is based on the assumption that the vectors are parallel and have the same magnitude and direction. If there are other conditions or information not included in the image, the answer might be different.
