There are many different ways to determine slope. The number of ways is much more than 2, but any particular method depends upon how much information about the line is known.
You seem to be asking about how slope is determined in two ways, using only the coordinates of the two given points.
Well, when I look at those coordinates, I immediately notice that the y-coordinates are equal. So, that tells me right away that the line passing through them has slope zero (because the y-coordinates are not changing).
So, that is one way.
Is using the Slope Formula another way? Well, technically, I suppose that it is, but the difference between the two methods seems trivial to me.
In the Slope Formula, the numerator is 3 - 3, so that's where the zero comes from.
I cannot think of two substantially-different ways to determine the slope of a horizontal line, given only the coordinates of two points on that line.
If we had more information (other than just coordinates of two point) about any line, there are certainly many different ways to determine slope.
You labeled the two given points C and D. Does this mean that you also have a diagram supplied with this exercise?
I'm thinking that you might have more given information (other than just the coordinates of two points) than you've shared with us.
Have you provided us with everything?
