You may have chosen this equation because its solution is $\displaystyle{x}={1}\usepackage{cmbright}\Huge\$ , or perhaps you substituted $\displaystyle-{1}\usepackage{cmbright}\Huge\$ in for x but incorrectly subtracted $\displaystyle-{1}\usepackage{cmbright}\Huge\$ from 10. However, when you substitute $\displaystyle{x}=-{1}\usepackage{cmbright}\Huge\$ into the left side you get $\displaystyle\sqrt{{{10}-{\left(-{1}\right)}}}=\sqrt{{{11}}}\usepackage{cmbright}\Huge\$ , which is not equal to 3. The correct answer is $\displaystyle\sqrt{{{2}{x}+{6}}}={2}\usepackage{cmbright}\Huge\$ .