You may have thought that the $\displaystyle-{25}\usepackage{cmbright}\Huge\$ and the 25 canceled out. Instead, you need 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.