#### Answer

$x=5$

#### Work Step by Step

RECALL:
(1) The slope-intercept form of a line's equation is $y=mx+b$ where $m$=slope and $(0, b)$ is the line's y-intercept.
(2) A line with an undefined slope is vertical, the equation of which is of the form $x=h$ where $h$=x-intercept. All points on the line have an x-coordinate of $h$.
The given line has an undefined slope therefore its equation is of the form $x=h$.
The line contains the point $(5, 1)$.
The x-coordinate of this point is $5$.
Thus, the line's equation is $x=5$.
The equation of the line cannot be written in slope-intercept form.