Appendix A - Review - A.4 Factoring Polynomials - A.4 Assess Your Understanding - Page A40: 71

$(y-3)^2$

To complete the square of $y^2 -6y$ we add $(b/2)^2$. In this case, the "$b$" is the second coefficient, $-6$, so we get $(6/2)^2=9$. That is, we have $y^2 -6y+9$. Then by factorization, we get $$ y^2 -6y+9=(y-3)^2 .$$