Add the equations to get: $\displaystyle{6}{x}={18}\usepackage{cmbright}\Huge\$

Solve for x to get $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ and substitute into the first equation:

$\displaystyle{5}{\left({3}\right)}+{4}{y}={23}\usepackage{cmbright}\Huge\$ , so $\displaystyle{4}{y}={8}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={2}\usepackage{cmbright}\Huge\$