$\,\,\,\,\,\,7x+3y\,\,\,\vdots \,\,\,13$
$\to 10\left( 7x+3y \right)\,\,\,\vdots \,\,\,\,13$
$\to 70x+30y\,\,\,\vdots \,\,\,13$
$\to 70x+30y-13.5x-13.2y\,\,\,\vdots \,\,\,13$
$\to 70x+30y-65x-26y\,\,\,\vdots \,\,\,13$
$\to 5x-4y\,\,\,\vdots \,\,\,13$