\[(x - 8)^{2} - 2x(6 - x)^{2} =\]
\[= x^{2} - 16x + 64 - 2x\left( 36 - 12x + x^{2} \right) =\]
\[= x^{2} - 16x + 64 - 72x + 24x^{2} - 2x^{3} =\]
\[= - 2x^{3} + 25x^{2} - 88x + 64.\]