You added 6 to both sides of the given equation. This produces a perfect square on the right side, but does not produce a perfect square trinomial on the left side. Remember that if you are given $\displaystyle{{x}}^{{2}}+{b}{x}\usepackage{cmbright}\Huge\$ , you must add $\displaystyle{{\left(\frac{{b}}{{2}}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ in order to produce a perfect square trinomial.