Slope judges the vertical angle in which the line is moving, from point to point. If the line is vertical in it of itself, there will be no specific point in which you can use to define the slope. The denominator of run is always 0, and having a 0 for denominator will result in the fraction being undefined. You cannot have parts of nothing, mathematically.
At one shop the coat was £24 with the 1/5 off and £30 in the other shop where it was 25% off so therefore the first coat she saw is the cheapest I think