The equation $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ describes a vertical line, so any line parallel to it will also be vertical. The equation $\displaystyle{x}=-{y}\usepackage{cmbright}\Huge\$ can be written as $\displaystyle{y}=-{x}\usepackage{cmbright}\Huge\$ , which is a line that goes down from left to right, so it is not vertical.