You may have chosen this equation because it has the same constant as $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ . However, $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ is a vertical line, so a line parallel to it must also be vertical. The equation $\displaystyle{x}={3}+{y}\usepackage{cmbright}\Huge\$ can be written as $\displaystyle{y}={x}-{3}\usepackage{cmbright}\Huge\$ , which is a line that goes up from left to right.