Secondly, this is to find the value of y (in layman's terms, it is related to the height of the coordinate point.

Method: You use a ruler, put it vertically and translate it from the left to the right of the figure. In the process of translation, the minimum value of y is found (that is, which of the three images is the lowest point), and of course the minimum value changes after each translation. Then find the maximum value among these. As for why, just look at other people's cases. But other people's theories are too strong, and I think the topic should be easy to understand. I believe this is the right way for me. I told some students.