You may have ignored the absolute value symbols and thought that $\displaystyle{\left|{4}+{\left(-{10}\right)}\right|}\usepackage{cmbright}\Huge\$ was equal to $\displaystyle-{6}\usepackage{cmbright}\Huge\$ . Remember that the absolute value of a negative number is positive, and therefore $\displaystyle{\left|{4}+{\left(-{10}\right)}\right|}={\left|-{6}\right|}={6}\usepackage{cmbright}\Huge\$ .