The slope of an horizontal line is always zero, it does not matter which point it passes thorugh.
This comes from the fact that in an horizontal line the y values of the points will always be the same, then from the definiton of the slope:
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]we notice that the numerator will be zero.
Therefore the slope is zero, that is:
[tex]m=0[/tex]