Answer
In this case you need to add $(\frac{b}{2})^{2}$ on both sides in that case you will have a perfect square.
$$(x+3)^{2}=9$$
Work Step by Step
What do you need to add to $ax^{2}+bx$ To complete the square?
In this case you need to add $(\frac{b}{2})^{2}$ on both sides in that case you will have a perfect square.
Complete the square for the expression $x^{2}+6x$.
$$x^{2}+6x$$ $$x^{2}+6x+(\frac{6}{2})^{2}=(\frac{6}{2}^{2}$$ $$x^{2}+6x+9=9$$ $$(x+3)^{2}=9$$
