This is one solution to the system, but not the only one. If you substitute $\displaystyle{2}{x}-{1}\usepackage{cmbright}\Huge\$ for y in the first equation and then simplify, you will end up with the equation $\displaystyle{10}{x}={10}{x}\usepackage{cmbright}\Huge\$ . This equation is true regardless of the value of x.