Answer
number to be added: $9$
factored form: $(y-3)^2$
Work Step by Step
RECALL:
To complete the square of an expression in the form $x^2-bx$, $(\frac{b}{a})^2$ must be added.
The factored form of the result is $(x-\frac{b}{2})^2$.
Thus, in the given expression add $(\frac{6}{2})^2=3^2=9$ to obtain:
$y^2 -6y + 9$
Writing the square in factored form gives:
$y^2-6y+9 = (y-3)^2$
