Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 31: 57

$x=2$

If the slope is undefined, then (by convention) the equation must be in the form $x=m$. All points on the line have the same $x$-coordinate. Thus, if the slope is of a line is undefined and the line contains $(2,4)$, the $x$-coordinate of which is $2$, then the equation of the line must be $x=2$.