This is a solution to the first equation, but not the second one, so it is not a solution of the system. 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\$ . Since this is false, you cannot find numbers that make both equations true.