If you solve a system and end up with the equations $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}=-{10}\usepackage{cmbright}\Huge\$ , then the solution is (0, $\displaystyle-{10}\usepackage{cmbright}\Huge\$ ). Here, if you substitute $\displaystyle{5}-{2}{y}\usepackage{cmbright}\Huge\$ for x in the second equation and then simplify, you will end up with the equation $\displaystyle{0}=-{10}\usepackage{cmbright}\Huge\$ , which means something very different. Since this is false, you cannot find numbers that make both equations true.