@MARS_999: uhhh... no. Let's clarify something: when you have (x, y) = P, x and y are not points. Together, they make up a single point, P. You need both of them to have one point.

2 points make up a line (in 2D our case, this requires 4 values). Once you have a line, you can find the slope (no additional points required).

If you do not have two points, you do not have a line. If you are missing one of the coordinate values, it is not a point (i.e. (x, ?) is not a point). You need two values to have a point, and you need two points (four values) to have a line, and you need a line to have a slope.

Your definition of "point" is off. A point in a 2d plane consists of two coordinates. Two such points define a line.