It appears that you used the wrong order of operations and solved the equation $\displaystyle{{x}}^{{2}}={36}\usepackage{cmbright}\Huge\$ . The correct first step is to subtract 4 from both sides to get $\displaystyle{{\left({x}-{4}\right)}}^{{2}}={32}\usepackage{cmbright}\Huge\$ . Take the square root of both sides, isolate x, and simplify the radical.