Even though $\displaystyle{y}={4}\usepackage{cmbright}\Huge\$ when $\displaystyle{x}={2}\usepackage{cmbright}\Huge\$ and $\displaystyle{y}={9}\usepackage{cmbright}\Huge\$ when $\displaystyle{x}={3}\usepackage{cmbright}\Huge\$ , this is a quadratic function, not a linear one.