Although the product of the two factors in this equation is equal to 0, this is not the same as x being equal to 0. If you substitute 0 for x in the left side of the equation, you will get $\displaystyle{{0}}^{{2}}-{4}\cdot{0}-{12}=-{12}\usepackage{cmbright}\Huge\$ , not 0. So $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ is not a solution.