I found this example in some old book and can't figure it out, why is this answer is right. Would like to get an explanation to it. Thanks in advance.

I don't have reputation over 10, so I uploaded image to some other host, hope it won't be a problem to you, here it is:
[![enter image description here](https://i.stack.imgur.com/Ugm9Z.png)](https://i.stack.imgur.com/Ugm9Z.png)
<http://postimg.org/image/auf1el5fz/>

and also sorry for tags, I don't know where is it belong