$\"\"$

Slope of the two non vertical lines is $\"\"$ .

The y-coordinates are same. So, the line is a horizontal line parallel to x-axis.

Slope of the line is zero. $\"\"$

Check the solution:

Replace $\"\"$ in slope formula.

$\"\"$

$\"\"$ (Subtract: $\"\"$ )

$\"\"$ (Divide: $\"\"$ ) $\"\"$

The slope is zero.