The blue diagonal line goes through the two points (0,2) and (5,-4). These are shown as the dark blue enlarged points. You can pick any other points you want that are on the diagonal line, though these are the easiest as they stand out the most.
Use the slope formula to find the slope through these points
m = (y2-y1)/(x2-x1)
m = (-4-2)/(5-0)
m = (-6)/(5)
m = -6/5
The negative slope means the line goes downhill as you move from left to right along the diagonal line.
