Final answer:
The coordinates for the fourth vertex of the parallelogram should be (2c, 2b).
Step-by-step explanation:
The given coordinates for the three vertices of the parallelogram are (0, 0), (2c, 0), and (2a, 2b). As one side of the parallelogram is along the x-axis, the fourth vertex must have the same y-coordinate as (2a, 2b), which is 2b. Therefore, the coordinates for the fourth vertex are (2c, 2b).