We plot the two points and draw the line:
We draw a horizontal green line from the left point until it is
exactly above the right point, like this:
We count the number of units which is the length of the green line.
We count 13. That number is called the RUN.
We draw a red vertical line from the right end of the green line
down to the line that we want to find the slope of, like this:
We count the number of units which is the length of the red line.
We count only 1 unit. But since it goes DOWN from the green line to the
given line, we make it NEGATIVE. So it is -1. That number -1 is called
the RISE.
The definition of slope is
Edwin