I do not understand this topic and particularly this example. In the class the result was found out but I still do not get it. How did the teacher came up with this outcome?

Firstly please show me what was the topic and the example in class. Given the topic I will provide the answer and the easiest way to complete it.
For example, let's say the student had to solve the equation 2x+7=0. I would show him step by step how to complete it and then check for understanding with an example of my own.