You should know that:
(a+b)(a+b)(a+b)
=a(a+b)(a+b)+b(a+b)(a+b)
=a(a²+2ab+b²)+b(a²+2ab+b²)
=a³+2a²b+ab²+a²b+2ab²+b³
=a³+3a²b+3ab²+b³
-----------------
If a=5x and b=-4y
(5x+(-4y))³
=(5x-4y)³
=(5x)³+3(5x)²(-4y)+3(5x)(-4y)²+(-4y)³
=125x³+3(25x²)(-4y)+15x(16y²)-64y³
=125x³+75x²(-4y)+240xy²-64y³
=125x³-300x²y+240xy²-64y³
