Explanation:
you mean the slope of the line between (8, 9) and (23, 9).
we can see that right away, as the y value does not change. it means it is a flat line, and that means the slope is 0.
now, formally :
the slope of a line is (y coordinate change / x coordinate change) when going from one point on the line to another.
the direction does not matter, but once we pick one for the comparison of one coordinate, we have to use the same direction also for the other.
I prefer to go from left to right.
so, from (8, 9) to (23, 9).
x changes by +15 (from 8 to 23).
y changes by 0 (from 9 to 9).
the slope is therefore 0/+15 = 0.