(1,-5) , (1,2)
y2 - y1
--------- = slope
x2 - x1
2 - (-5)
---------- = slope {substituted coordinates into slope formula}
1 - 1
7
---- = slope {simplified on top and bottom}
0
slope = undefined {you cannot divide by zero, that is "undefined"}
Also, it is a vertical line, because vertical lines have slopes which are undefined.
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