Answer:
(9x - 8y)(81x² + 72xy + 64y²)
Explanation:
given
729x³ - 512y³
this is a difference of cubes and factorises in general as
a³ - b³ = (a - b)(a² + ab + b² )
then
729x³ - 512y³
= (9x)³ - (8y)³ ← with a = 9x and b = 8y
= (9x - 8y)((9x)² + 9x(8y) + (8y)² )
= (9x - 8y)(81x² + 72xy + 64y² ) ← in factored form