To factor the trinomial, you can use the ac method where you multiply the first and last terms together (6 and 9) and add up to 15. In this case ac= 54 2 numbers that multiply to 54 and add up to 15 would be 6 and 9 so place these numbers where the 15 would be: Then group terms to simplify: Simplify any like terms: And group together again: You can simplify a 3 out of the first expression one more time to get: Which would be the factored form. I hope this helps :)
With the given information you can form a system of equations with the help of the formular for the nth term of an arithmetic sequence. From which you find the first term and the common difference, with which you can wite the recursive formula for the sequence.