i can't understand  the  solution of example 3rd, please explain me  in easier way

Example 3 is explained by teacher video in the following link:

https://www.meritnation.com/study-online/study-material/5jSn3YKBeaKCJB6vpKrnjg!!/Zch6bNaFNN$pjJ0BsGgAvg!!

Please go to the second Video(s) by teachers.

If you still face any problem then do get back to us.