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 first equation by $\displaystyle-{4}\usepackage{cmbright}\Huge\$ and add it to the second equation, then you end up with $\displaystyle{0}={0}\usepackage{cmbright}\Huge\$ , which means something very different, and that is true regardless of the value of x or y.