If you solve a system and end up with the equations $\displaystyle{x}={10}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={11}\usepackage{cmbright}\Huge\$ , then the solution is $\displaystyle{\left({10},{11}\right)}\usepackage{cmbright}\Huge\$ . Here, if you substitute $\displaystyle{2}{x}+{5}\usepackage{cmbright}\Huge\$ for y in the second equation and then simplify, you will end up with the equation $\displaystyle{10}={11}\usepackage{cmbright}\Huge\$ , which means something very different. Since this is false, you cannot find numbers that make both equations true.