The formula for slope is [tex] \frac{y_2-y_1}{x_2-x_1} [/tex] where x₁=-18, y₁=-20, x₂=-18, y₂=-15. We can quickly see a problem that both x values are -18, which implies that this is a vertical line. Since both x values are the same, which go on the bottom and result in a denominator of 0, the slope is undefined
