As we know the formula to calculate the slope is m = y2 - y1 divided by x2 - x1.
Where m is the slope. y2 is the second y co ordinate. y1 is the first y co ordinate. x2 is the second x co ordinate. x1 is the first x co ordinate.
So to calculate we can write it out as m = -3 - (-1) / -1 - (-1) which becomes = -3 + 1/ -1 + 1. As you can see the slope of this line is undefined. This means the line is vertical.