You may have squared both sides, simplified, and found the solution $\displaystyle{x}={4}\usepackage{cmbright}\Huge\$ to the resulting quadratic equation. However, there are two solutions to that quadratic equation, and you must go back to the original equation and check them. You will find that $\displaystyle{x}={4}\usepackage{cmbright}\Huge\$ is an extraneous solution.