Substituting 7 for x and 4 for y in each equation is the correct first step to tell if an ordered pair is a solution to a system.

$\displaystyle{x}-{y}={3}\usepackage{cmbright}\Huge\$ $\displaystyle{y}={2}{x}-{10}\usepackage{cmbright}\Huge\$

$\displaystyle{7}-{4}={3}\usepackage{cmbright}\Huge\$ $\displaystyle{4}={2}{\left({7}\right)}-{10}\usepackage{cmbright}\Huge\$

$\displaystyle{x}-{y}={3}\usepackage{cmbright}\Huge\$	$\displaystyle{y}={2}{x}-{10}\usepackage{cmbright}\Huge\$
$\displaystyle{7}-{4}={3}\usepackage{cmbright}\Huge\$	$\displaystyle{4}={2}{\left({7}\right)}-{10}\usepackage{cmbright}\Huge\$