Final answer:
The y-coordinate of point T is -4.5, as moving 4.5 units down from the origin on a Cartesian coordinate system results in a negative y-coordinate.
Step-by-step explanation:
The y-coordinate of point T can be found by moving point T vertically relative to the origin of a Cartesian coordinate system. Given that point T is 4.5 units down from the origin, the movement downwards from the origin is in the negative direction on the y-axis. Therefore, the y-coordinate of point T is -4.5. Since the question does not specify a starting point, we assume the starting point is the origin (0,0), and moving 4.5 units down from there results in the point having a y-coordinate of -4.5.