What is the x coordinate of the ordered pair?

Answer 1

Usually the first of the two numbers that comprise the ordered pair.

However, in the case of a displacement-time graph, the first is usually the t-coordinate (time) and the x-coordinate is the second (displacement).

Answer 2

Usually, it is the first of the ordered pair and gives the perpendicular distance of the point from the y-axis. In 2-dimensions, this would be the horizontal distance from the vertical axis.