The slope intercept is equal to rise/run.
From the y intercept (0, 4) you must go down 1 unit to reach point (0, 3).
Then you have to go right 2 units to get to point (2, 3).
Since you went down 1 unit, the rise is -1.
Since you went to the right 2 units, the run is positive 2.
-1/2 is the slope intercept.
If you had gone up one unit, then the rise would have been positive 1. If you had gone to the left 2 units, then the run would have been -2.