[![enter image description here](https://i.stack.imgur.com/Ji9zb.png)](https://i.stack.imgur.com/Ji9zb.png)

I have tried to undersatnd how this step works, it shows a rather messy equation on the top line. Then the line below is one step taken to simplfy. But I can't quite figure out what has happend.

If anyone could shed some light on this it would be extremely helpful.