When simplifying, you may have forgotten that a negative times a positive is negative.

$\displaystyle{3}{\left({3}\right)}={2}{\left(-{4}\right)}+{1}\usepackage{cmbright}\Huge\$ or $\displaystyle{9}=-{8}+{1}\usepackage{cmbright}\Huge\$ or $\displaystyle{9}=-{7}\usepackage{cmbright}\Huge\$

This is not a true statement, so the ordered pair $\displaystyle{\left({3},-{4}\right)}\usepackage{cmbright}\Huge\$ is not a solution to the equation.