Add the equations to get: $\displaystyle{10}{y}={40}\usepackage{cmbright}\Huge\$

Solve for y to get $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ and substitute into the first equation:

$\displaystyle-{2}{x}+{4}=-{8}\usepackage{cmbright}\Huge\$ , so $\displaystyle-{2}{x}=-{12}\usepackage{cmbright}\Huge\$ and $\displaystyle{x}={6}\usepackage{cmbright}\Huge\$