(a² + b²) /(a³ - b³) - 2/(3a - 3b) = (a² + b²) / [(a - b)(a² + ab + b²) - 2 / [3(a - b)] =
= (3a² + 3b²- 2a² - 2ab - 2b²) / [3(a - b)(a² + ab + b²)] =
= (a² - 2ab + b²) / [3(a - b)(a² + ab + b²)] = (a - b)² /[3(a - b)(a²+ab+b²)] =
= (a-b) /[3(a² + ab+b²)]