OKAY. I realized where the 2 came from. I don't know where the -4 comes into play still, though.
Why are we subtracting here?
Continuing...it says though before what's below:
After adding and subtracting 4 within the parentheses, you must now regroup the terms to form a perfect square trinomial. The -4 can be removed from inside the parentheses (4-4 is zero...why is the -4 there, what is the point if there's still going to be a 4 there somehow untouched); however, because of the 2 outside of the parantheses (where did that come from?), you must multiply by -4 by 2, as shown below.