It appears that you used the wrong order of operations and solved the equation $\displaystyle{{\left({x}-{21}\right)}}^{{2}}={25}\usepackage{cmbright}\Huge\$ . The correct first step is to add 25 to both sides to get $\displaystyle{{\left({x}+{4}\right)}}^{{2}}={50}\usepackage{cmbright}\Huge\$ . Take the square root of both sides, isolate x, and simplify the radical.