To find the slope between two points, we have the formula below:
![m=(y_2-y_1)/(x_2-x_1)](https://img.qammunity.org/2023/formulas/mathematics/high-school/78uaqhwt0aws3qfwxigaftpihnmb1gzxtp.png)
where m = slope and x and y are the coordinates of the two points.
In the question, we have (x₁, y₁) = (-9, 9). Our (x₂. y₂) is (-3, 9).
Let's plug this in to the formula above.
![m=(9-9)/(-3-(-9))=(0)/(-3+9)=(0)/(6)=0](https://img.qammunity.org/2023/formulas/mathematics/college/f13n2kfrvzx0m3jjsgkii423sa0dtk1ail.png)
Therefore, the slope of the line passing two points (-9, 9) and (-3, 9) is zero. This line is a horizontal line since both have the same y-coordinates.