[tex]-4x^2 +64x +256 =0\\\\\implies x^2 -16x -64 =0\\\\\implies x^2 - 2 \cdot 8 \cdot x + 8^2 -8^2 -64=0\\\\\implies (x-8)^2 -128 =0\\\\\implies (x-8)^2 = 128\\\\\implies x-8 = \pm\sqrt{128} = \pm \sqrt{64 \times 2} = \pm 8\sqrt 2\\\\\implies x = 8 \pm 8\sqrt2\\\\\text{Hence,}~ x = 8+ 8\sqrt 2~\text{or}~ x = 8 - 8\sqrt 2[/tex]
