Find the slope of a line going through points (-4,3) and (5,3) by calculating the ratio of the change in y to the change in x.
To do this I like to line up the points and subtract (this is finding the change). After doing so, you will put 2nd difference over 1st difference.
(-4 , 3)
-(5 , 3)
------------
-9 0
; this reduces to 0.
The slope is 0.
Slope is rise over run.
So if you had looked at the y's there was no change (0 change) while there was change in the x's. This means the line is horizontal since the graph isn't rising.