This is what i got in the caculator 6400 if not im so so sory
K is the slope of the line formed by connecting the points.
Use slope formula:

where the 2 points are the end points of the line (First and last going Left to right)

Substitute into slope formula

You can check if this is correct by going back to graph and going "up 7" and "over 2" to get from one point to the next.
The answer to your question is -3 !!