23.9. a) (x² - xy + y²)(x + y);

Recognize the sum of cubes formula: a³ + b³ = (a + b)(a² - ab + b²).

1. In this case, a = x and b = y.
2. Therefore, (x² - xy + y²)(x + y) = x³ + y³.

Answer: x³ + y³