It is known that,
[a^3 - b^3 = \left( a - b \right)\left( a^2 + b^2 + ab \right)]
[\ 64 x^3 - 729 y^3 ]
[ = \left( 4x \right)^3 - \left( 9y \right)^3 ]
[ = \left( 4x - 9y \right)\left{ \left( 4x \right)^2 + \left( 9y \right)^2 + \left( 4x \right) \times \left( 9y \right) \right}]
[ = \left( 4x - 9y \right)\left( 16 x^2 + 81 y^2 + 36xy \right)]
