The constant that you need to add to complete the square is always a positive number. If you start with an expression of the form $\displaystyle{{x}}^{{2}}+{b}{x}\usepackage{cmbright}\Huge\$ , you need to add the number $\displaystyle{{\left(\frac{{b}}{{2}}\right)}}^{{2}}\usepackage{cmbright}\Huge\$ to complete the square.