If you end up with an equation that is always true, such as $\displaystyle{x}={x}\usepackage{cmbright}\Huge\$ , then there are an infinite number of solutions. However, 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}={5}\usepackage{cmbright}\Huge\$ . Since this is false, you cannot find numbers that make both equations true.