If you solve a system and end up with the equations $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}=-{4}\usepackage{cmbright}\Huge\$ , then the solution is $\displaystyle{\left({0},-{4}\right)}\usepackage{cmbright}\Huge\$ . Here, if you multiply the second equation by $\displaystyle-{7}\usepackage{cmbright}\Huge\$ and add it to the first equation, you will end up with the equation $\displaystyle{0}=-{4}\usepackage{cmbright}\Huge\$ , which means something very different. Since this is false, you cannot find numbers that make both equations true.