After many, many years of teaching, this is still one of my least favorite topics to teach. It is definitely one that the day you do it, almost all kids “master” the ideas very quickly. But, even high level students will miss the boat and give incorrect equations for vertical and horizontal lines. It discourages me because equations to me simply describe points to be plotted. So, the equation x=5 is straightforward to me and is clearly a vertical line. Please note that I do not cover the slopes of these two lines. Their equations do not arise in the Common Core standards, so it is difficult to know how deeply to cover them.