(2x-5y)³-(2x+5y)³
=8x³-125y³-3*2x*5y(2x-5y)-[8x³+125y³+3*2x*5y(2x+5y)]
=8x³-125y³-3*2x*5y(2x-5y)-8x³-125y³-3*2x*5y(2x+5y)]
=-250y³-60x²y+150xy²-60x²y-150xy²
=-250y³-120x²y
or
(a-b)³-(a+b)³=-2b³-6(a²b)
=-2(5y)³-6(2x*2x*5y)
=-2(125y³)-120x²y
=-250y³-120x²y
= -(250y³+120x²y)