Substituting 3 for x and 1 for y in each equation is the correct first step to tell if an ordered pair is a solution to a system.

$\displaystyle{y}+{x}={4}\usepackage{cmbright}\Huge\$ $\displaystyle{y}={2}{x}-{5}\usepackage{cmbright}\Huge\$

$\displaystyle{1}+{3}={4}\usepackage{cmbright}\Huge\$ $\displaystyle{1}={2}{\left({3}\right)}-{5}\usepackage{cmbright}\Huge\$

$\displaystyle{y}+{x}={4}\usepackage{cmbright}\Huge\$	$\displaystyle{y}={2}{x}-{5}\usepackage{cmbright}\Huge\$
$\displaystyle{1}+{3}={4}\usepackage{cmbright}\Huge\$	$\displaystyle{1}={2}{\left({3}\right)}-{5}\usepackage{cmbright}\Huge\$