Appendix A - Review - A.4 Factoring Polynomials - A.4 Assess Your Understanding - Page A40: 72

$4$, $(x-2)^2$

To complete the square of $x^2 -4x$, we add $(b/2)^2$. In this case, the "$b$" is the second coefficient, $-4$, so we add $(-4/2)^2=4$ and get $x^2 -4x+4$. Then by factorization, we get $$ x^2 -4x+4=(x-2)^2 .$$