What is the slope of a vertical line?

Answer: Undefined

Explanation: All the X values are 0, therefore anytime you have a number that is over 0 (7/0, 6/0 etc.), the slope is undefined.

The slope of a vertical line does not exist! We can't divide by zero, which is of course why this slope value is "undefined". This relationship is always true: a vertical line will have no slope, and "the slope is undefined" or "the line has no slope" means that the line is vertical.