Use the rise/run formula.
Choose one point to be the first point (-1,-3), and the other to be the second point (-1,-1).
(y2 - y1) / (x2 - x1) = slope of line.
(-1 - (-3)) / (-1 - (-1)) =
(2 / 0) = undefined!
This line is a completely vertical line; it has no slope.
