remember slope is just rise (change in y) over run (change in x) and since there's no change it has a zero slope, which applies to any horizontal line parallel to the x axis
(this is just me trying to explain slopes, separate from the question you put) if the right most point was 1 higher (5,7) instead of (5,6) then the slope would be 1/2 because to get from (5,7) to (3,6) you move up 1 and over 2
