Slope is rise/run.
y=-1 is a horizontal line where the y value is always one no matter what x is.
Therefore the rise is 0 and run is infinite
so 0/infinity=0 because 0/anything=0.
One thing I want to note is if you have a vertical line, where rise is infinite, and run is 0 infinite/0=undefined because you can't divide by 0 :-)