If you solve a system and end up with the equations $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}=-{2}\usepackage{cmbright}\Huge\$ , then the solution is $\displaystyle{\left({0},-{2}\right)}\usepackage{cmbright}\Huge\$ . Here, if you multiply the first equation by 2 and add it to the second equation, you will end up with the equation $\displaystyle{0}=-{2}\usepackage{cmbright}\Huge\$ , which means something very different.