She's right BUT!!! -- how in the lulu was she 
able to go from

this step:



to this step????:



It is correct but she doesn't explain how she did that.
I have no idea how she did that myself!  A magic wand!  :)

------------------------------

I would not have multiplied everything out as she did.
Here's what I would have done:



I'd multiply  out as 



Then I'd realize that if we factor  out of 
we get  which is 2 times the 2x(x+1) in the 
parentheses in the first term on the right:



Now to make things simpler, I will let u = 2x(x+1)
then 4x(x+1) will equal to 2u, and we have:



Taking positive square roots



And since , we have:





Edwin