$$( 8 x + 8 y ) ^ { 2 } - ( 8 x - 8 y ) ^ { 2 } - 128 x y$$
$128xy$
$$64x^{2}+128xy+64y^{2}-\left(8x-8y\right)^{2}-128xy$$
$$64x^{2}+128xy+64y^{2}-\left(64x^{2}-128xy+64y^{2}\right)-128xy$$
$$64x^{2}+128xy+64y^{2}-64x^{2}+128xy-64y^{2}-128xy$$
$$128xy+64y^{2}+128xy-64y^{2}-128xy$$
$$256xy+64y^{2}-64y^{2}-128xy$$
$$256xy-128xy$$
$$128xy$$
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