8x out of nowhere?

I'll admit it now: I understand most math and the concepts behind it. Regardless, I have much to improve upon when it comes to applying it and often make lots of stupid little mistakes.

So in this problem, can someone please tell me where the 8x comes from in the third line? It makes absolutely no sense to me. I'm sure it's another obvious detail I'm overlooking, lol.

http://c811129.r29.cf2.rackcdn.com/se0ae01135.png

Thanks in advance!

(X-4)^2=x^2-8x+16

Hello, zsf1990!

Don't tell me you think that is equal to !

Have you forgotten your Algebra?

. .

Multiply it out!

I wasn't treating the right hand side as a factor. Like I said, foolish mistakes. That doesn't change how stupid I feel...