Substitute 8 for x and $\displaystyle-{1}\usepackage{cmbright}\Huge\$ for y.

$\displaystyle{8}+{3}{\left(-{1}\right)}={11}\usepackage{cmbright}\Huge\$	$\displaystyle-{1}={8}-{7}\usepackage{cmbright}\Huge\$
$\displaystyle{8}+{\left(-{3}\right)}={11}\usepackage{cmbright}\Huge\$	$\displaystyle-{1}={1}\usepackage{cmbright}\Huge\$ False
$\displaystyle{5}={11}\usepackage{cmbright}\Huge\$ False

Since the ordered pair does not make either equation true, it is not a solution to the system. To be a solution to the system it needs to make both equations true.