Question: what is the equation of a line when the slope is undefined?
Answer: 1 Answer. If the slope of a line is undefined, then the line is a vertical line, so it cannot be written in slope-intercept form, but it can be written in the form: x=a , where a is a constant. If the line has an undefined slope and passes through the point (2,3) , then the equation of the line is x=2 .