You may have subtracted 17 from both sides of the equation. This will give you an equivalent equation, $\displaystyle{x}-{23}={0}\usepackage{cmbright}\Huge\$ . However, this does not mean that x is equal to $\displaystyle-{23}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{x}={23}\usepackage{cmbright}\Huge\$ .