(2/3x - 3y)^3 = (2/3x - 3y)(2/3x - 3y)(2/3x - 3y)
= (4/9x^2 - 4x 3y + 9y^2)(2/3x - 3y)
= 8/27x^3 - 8/3x^2 3y + 12x 9y^2 - 4x 9y^2 - 27y^3
= 8/27x^3 - 8x^2y + 108xy^2 - 36xy^2 - 27y^3
= 8/27x^3 - 8x^2y + 72xy^2 - 27y^3.