en.wikipedia.org/wiki/Completing_the_square

In elementary algebra, **completing the square** is a technique for converting a
quadratic polynomial of the form. a x 2 + b x + c {\displaystyle ax^{2}+bx+c} ...

www.mathsisfun.com/algebra/completing-square.html

But if you have time, let me show you how to "**Complete the Square**" yourself.
**Completing the Square**. Say we have a simple expression like x^{2} + bx. Having x
...

www.khanacademy.org/math/algebra/quadratics/solving-quadratics-by-completing-the-square/v/solving-quadratic-equations-by-completing-the-square

For example, x²+6x+5 isn't a perfect square, but if we add 4 we get (x+3)². This, in
essence, is the method of ***completing the square***. Created by Sal Khan, ...

www.khanacademy.org/math/algebra/quadratics/solving-quadratics-by-completing-the-square/a/solving-quadratic-equations-by-completing-the-square

For example, solve x²+6x=-2 by manipulating it into (x+3)²=7 and then taking the
**square** root.

www.purplemath.com/modules/sqrquad.htm

Demonstrates, with step-by-step instructions and illustrations, how to **complete**
**the square** to solve a quadratic equation.

www.mathplanet.com/education/algebra-1/quadratic-equations/completing-the-square

we can't use the **square** root initially since we do not have c-value. But we can
add a constant d to both sides of the equation to get a new equivalent equation ...