when the slope is undefined, meaning rise/run has a 0 at the bottom namely rise/0.
in such cases, when the run = 0, it's just a vertical line, and the equation will be x = some number, like x = 3 or such.
using the point-slope form, say the line passes through (3,5) and (3, 13), then

now, when the slope is zero, it means a horizonta line, namely rise/run has a rise = 0, so is really 0/run, and we usually write that as y = some number, say y = 7.
say the line passes through (3, 7) and (8 , 7)
