i too cant understand the example 2.please tell me.

The said example is as below:

Here, we have to find the shortest path followed by Sanjana from P to Q (including touching the wall). 

To find this path, we used the concept of mirror image. Image of any object in the mirror is as far from the mirror as the object.

Let the wall be a mirror and the image of Sanjana be at P' behind the wall, so the distances of both points P and P' will be same from the wall. It can be understood from the below given figure.