How many ways are there to make an octagon with 19 different sticks when order matters

Describe a pattern in each sequence what are the next two terms of each sequence -5, 5, -5, 5 a. subtract 10 from the previous term; -5,5 b. multiply the previous term by 1; 5, 5 c. add 10 to the previous term; -5, -15 d. multiply the previous term by -1; -5, 5