We are given the following two points
[tex](-4,9)\; and\; (4,9)[/tex]
Notice that the y-coordinates are the same meaning that it is a horizontal line.
Let us find the slope of the line passing through these points
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]
Substitute the given points into the above formula
[tex]m=\frac{y_2-y_1}{x_2-x_1}=\frac{9-9}{4-(-4)}=\frac{0}{4+4}=0[/tex]
As you can see, the slope of a horizontal line is always 0.
Now, consider the following two points
[tex]undefined[/tex]
