The midpoint of this number line segment is the average of the endpoints.
(-9+2)/2=-7/2=-3½.
Each coordinate of a point is actually a number line. If we have two coordinates, that is, two axes (labelled x and y) each axis is a number line, and the number lines are at right angles to each other and they cross at the coordinates (0,0), that is, x=0 and y=0 (the centre point of each number line which has no limit to its endpoints. One endpoint goes to plus infinity and the other endpoint to negative infinity).