(0, -5)
Graph these points so that the center of the clock is at (0, 0) and 12 is at (0, 5).
You know that, on a clock, 6 is directly below point 12 (0, 5). Therefore, we know that point 6 will have an x coordinate of 0.
Point 6 will also be the same distance from the center that point 12 is. Note that point 12 is 5 units from the center. Thus, point 6 will also be 5 units from the center.
There are two points that have an x coordinate of 0 and are 5 units from the center (0, 0). These are (0, 5) and (0, -5). Since the first of these is point 12, we know that (0, -5) must be point 6.