Just a quick check if someone wouldn't mind (different problem):
as for completing the square... still trying to burn that into my brain with no luck.
To work backwards from that, you examine the multiplier of x and take half of it...
x is multiplied by 2b, so half of that is b, hence we need to introduce by adding to both sides
Now the left side is a "multiplied out" square
so you can continue by taking the square root of both sides as before