The slope is the “change in y over the change in x,” also referred to as “rise over run.”
Pick any two points and calculate the difference in their y-values and divide that by the difference in their x-values.
Notice that all y-values are the same, so your “change in y” or your “rise” is always 0.
0/anything = 0 (as long as the denominator isn’t 0)
So, your slope here is 0.
