Hey there!
When you have a line that is set equal to y, it will be horizontal. While you could technically solve for the slope of this line, horizontal lines will always remain 0.
I'll show you what I mean with two example points from this line:
(0, 3)(2, 3)
x1 = 0, y1 = 3
x2 = 2, y2 = 3
y2 – y1 / x2 – x1
3 – 3 / 2 – 0
0 / 2 = 0
This will remain 0 no matter which points on any horizontal line you do.
Hope this helped you out! :-)