If you solve a system and end up with the equations $\displaystyle{x}={12}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={12}\usepackage{cmbright}\Huge\$ , then the solution is $\displaystyle{\left({12},{12}\right)}\usepackage{cmbright}\Huge\$ . Here, if you substitute $\displaystyle{3}{x}+{1}\usepackage{cmbright}\Huge\$ for y in the first equation and then simplify, you will end up with the equation $\displaystyle{12}{x}={12}{x}\usepackage{cmbright}\Huge\$ . This equation is true regardless of the value of x.