Because it says so in the Odyssey. Both Teiresias in Book 11 and Circe in Book 12 warned Ulysses that the cattle on the island of Thrinacian belongs to Helios, and that if any of them are killed then Ulysses will lose his ship and his men. But both books 11 and 12 are part of the Story beginning in Book 9 where Ulysses tells the Phaeacians about his adventures. And we know at the time that Ulysses is telling the story that he had already lost his ship and crew. Therefore we know that Helios's cattle will be slain before we are later told about their slaughter in Book 12.
Because we need Mc Doubles.
thanks for the answers EVERYONE xx