Answer: The slope is -1/2
Start at any point on the blue line. The best place in my opinion is the y intercept. This is the location (0,3)
Start at this location and move 1 unit down, then move 2 places to the right to land on the point (2,2)
The fact we moved down 1 unit means the rise is -1. So rise = -1. The run is 2 since we moved 2 spots to the right. So run = 2.
Divide the rise and run: slope = rise/run = -1/2
As a decimal, -1/2 = -0.5 but I would keep the slope as a fraction so you can see the rise and run easier.
note: the slope is negative because we go downhill as you move from left to right on the blue line.