What is the slope of a horizontal line and a vertical line?

Answer 1

a horizontal line has zero slope, and a vertical line has undefined slope. A way to remember this is when a skier is going down a horizontal line there is zero slope, but when he goes down the vertical line then it is undefined what will happen to him. =P

Answer 2

Think on a graph what it means to be a horizontal line. That means the value doesn't go up or down in the y direction. It's constant. . .

For a vertical, the value doesn't go up or down in the x direction.