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Explanation:
Let's apply the slope formula
m = (y2-y1)/(x2-x1)
m = (-7-(-3))/(-3-(-3))
m = (-7+3)/(-3+3)
m = -4/0
We run into trouble because 0 is in the denominator. We cannot divide by zero. This is why the slope is undefined.
The two points (-3,5) and (-3,-7) form a vertical line when you draw a line through them. All vertical lines have an undefined slope.
Put another way: the x coordinates of both points are the same (both are -3), so when we subtract those x coordinates, we end up with 0 in the denominator that leads to an undefined slope.
