You may have done a horizontal line test instead of a vertical line test. If you draw a vertical line through any point on the graph with $\displaystyle{x}=-{3}\usepackage{cmbright}\Huge\$ or $\displaystyle{x}={1}\usepackage{cmbright}\Huge\$ , you will find that the line passes through more than one point. Therefore, the graph fails the vertical line test and does not represent a function.