The line passes through two points that have the same x-coordinate.
It is a vertical line. To find the slope of a line, use any two points. Subtract the y-coordinates. Subtract the x-coordinates in the same order. Then divide the difference of the y-coordinates by the difference of the x-coordinates. Since in this case, the x-coordinates are both -6, the difference between the x-coordinates is zero. Division by zero is not defined, so the slope of this line is undefined. You can't write its equation in point-slope form, because there is no slope for this line.