If you solve a system and end up with the equations $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={0}\usepackage{cmbright}\Huge\$ , then the solution is (0, 0). Here, if you multiply the second equation by $\displaystyle-{6}\usepackage{cmbright}\Huge\$ and add it to the first equation, then you end up with $\displaystyle{0}={0}\usepackage{cmbright}\Huge\$ . This is true regardless of the value of x or y.