If a line has an undefined slope, what can you say about the horizontal coordinates of each of the points on the line?

an undefined slope is a vertical line, so this means that every point on the line has the same x-coordinate (horizontal coordinate)

Dr. Calc

The above answer is correct.

However, here is a different approach.

Let's think about what it means to have undefined slope. For any two points on a straight line (x1,y1) and (x2,y2), the slope of the line is defined as SLOPE = (y2-y1)/(x2-x1), except that if x1=x2 then the denominator is zero and the result is undefined. This is because division by zero is not a defined operation; it's just not allowed. x1=x2 occurs on a vertical straight line. That is why the slope is undefined.