This equation can be represented through a,b,c by ax^2 + bx + c. To find the value we want in the final square, we divide the b value by two (in this case 7/2). We then replace the c value with (b/2)^2.
We then pull the original c value out with parenthesis, and subtract the new c value. That is then added after the parenthesis.
You then replace the parenthesis with (x - b/2)^2
Hope this helped.