an equation that has an undefined slope is a vertical line. A vertical line is represented by x = a number. You cannot write an equation for a vertical line in slope intercept form (because of the undefined slope)...but u can write it in standard form.
lets say u have a vertical line going thru (2,3).....since it is a vertical line, its equation would be x = 2
to write this in standard form, it would be : x + 0y = 2 <==