slope=rise/run
when rise=0 or when it is a horizontal line (y=something, a line that goes straight left and right) then you get a zero slopoe
when run=0, or when it goes up and down infinitely or vertical line (x=something) you get undefined slope because rise/0 is undfined
zero slope would be y=c where c is a constant
undefined slope would be x=c where c is a constant
zero slope is a horizontal line
undefined slope is a vertical line
