A line that is hotizontal in the coordinate system, that is, the line is parallel to the x-axis has a slope equal zero.
For example y you look at the general equation in slope intersect form:
[tex]\begin{gathered} y=mx+b \\ \text{For m=0} \\ y=0x+b \\ y=b \end{gathered}[/tex]As you can see, the variable y is equal to the a constant. This means that regardles the value of x, y will always have the same value.
Since y always has the same value regardles the value of x, using the given point of the line (9,-7) we can determine the equation for the line as:
[tex]y=-7[/tex]
