Which option shows the correct way to begin showing that $\displaystyle{\left({7},{4}\right)}\usepackage{cmbright}\Huge\$ is a solution to the following system?

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

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