You may have isolated the radical, but then squared the left side and not the right side, to get the incorrect equation $\displaystyle{8}{x}+{1}={2}{x}-{5}\usepackage{cmbright}\Huge\$ , which would have the solution of $\displaystyle-{1}\usepackage{cmbright}\Huge\$ . Make sure that you always perform the same operation on both sides of an equation. In this case, you will get $\displaystyle{8}{x}+{1}={4}{{x}}^{{2}}-{20}{x}+{25}\usepackage{cmbright}\Huge\$ . Solve this quadratic equation and check the solutions back in the original equation.