4) Приведем подобные члены: $$6t^3v^5 - 8t^5v^3 + 3t^3v^5 = (6t^3v^5 + 3t^3v^5) - 8t^5v^3 = 9t^3v^5 - 8t^5v^3$$
Ответ: $$9t^3v^5 - 8t^5v^3$$