You probably made a mistake with signs.

Substitute $\displaystyle{x}=-{3}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={2}\usepackage{cmbright}\Huge\$ into the equation: $\displaystyle{2}{\left(-{3}\right)}-{7}{\left({2}\right)}={20}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle-{6}-{14}={20}\usepackage{cmbright}\Huge\$ or $\displaystyle-{20}={20}\usepackage{cmbright}\Huge\$ .

This is not a true statement, so the ordered pair $\displaystyle{\left(-{3},{2}\right)}\usepackage{cmbright}\Huge\$ is not a solution to the equation.