Simplifying matrix equation

Hey folks. I'm having trouble understanding the transition from line three to line four, in particular where the part gets to. When I tried it I get what's in line four, but with as an additional term. What am I missing here?

http://i.imgur.com/pQyX5.png

Re: Simplifying matrix equation

Can nobody help? To add more context, the result is a real number, which is why line three is identical when the trace is taken.

Re: Simplifying matrix equation

I think I just realised what it is. They took the derivative of that term, which becomes 0. Silly me.