This is a graph of the equation $\displaystyle{y}={3}{{x}}^{{2}}.\usepackage{cmbright}\Huge\$ Because there are other terms in the equation besides $\displaystyle{3}{{x}}^{{2}}\usepackage{cmbright}\Huge\$ , you should expect that the value for $\displaystyle{y}\usepackage{cmbright}\Huge\$ is not zero when $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ .