Find an equation of the line that passes through the given point and has the indicated slope:
(6,-1), m = undefined

"Undefined" slope means the line on the graph is vertical, and that
tells the whole story right there.  The 'x' coordinate is the same for
every point on the line.  If  (6, -1)  is a point on the line, then you know
that the x-value of EVERY point on it is  6 . 

It's a vertical line that intersects the x-axis at  x=6,  and the equation
of the line is  x=6 .

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