I don't know if there's an easier way to do this but this is what I came up with. We can break this hexagon into 2 triangles and a rectangle (the 1st triangle using the top 3 points, the 2nd using the bottom 3, and the rectangle using the "middle" ones).
If the slope is zero that means that the line does not go up (or have any rise) and therefore must be a horizontal line. Since it passes through the point (3,-8), the equation has to be y=-8.
