This is because the x coordinates of each point are the same (both are 3). These two points form a vertical line, and any vertical line has an undefined slope.
We could use the slope formula to see that...
m = (y2-y1)/(x2-x1)
m = (1-2)/(3-3)
m = -1/0
m = undefined
We cannot divide by zero, so that's why the slope is undefined.
