Using Two Points to Find Slope Page 4 of 5
A Slope of ZeroA line with a slope of zero is just a flat, horizontal line. Its rise is 0 because it's not going up or down from left to right. If you pick any two points on a horizontal line, they will have the same yvalue. If you plug the points into the slope formula, you'll end up with a zero in the numerator. 0 divided by any nonzero number is just equal to 0.
Example 

Find the slope of the line through the points (1,2) and (4,2).
Step 1: Label the points. It doesn't matter which one you pick as "Point 1" and "Point 2." Remember the x's are listed first in an ordered pair and the y's are listed second.
Step 1: Label the points. It doesn't matter which one you pick as "Point 1" and "Point 2." Remember the x's are listed first in an ordered pair and the y's are listed second.
Step 2: Plug the values into the slope formula. Subtract the y's on the top, subtract the x's on the bottom. Make sure to subtract in the same order in the numerator and denominator.
Step 3: Make sure the answer is simplified. In this case, 0 divided by 3 is just 0. Zero divided by any nonzero number will always be 0.
Take a look at this on a graph to verify the answer. The points (1,2) and (4,2) have the same yvalues and form a horizontal line. We got a 0 in the numerator when we used the slope formula because the rise was 0. The line didn't go up or down from left to right. The value for the run doesn't really matter because 0 divided by any nonzero number is still 0.