Question

If a sub 1 is equal to -35 and s sub 15 is equal to 420 what is a sub 15?

Answer 1

no idea. what is the relation between the "a" terms and the "s" terms?

If the "s" is a typo and you meant "a", then why do you tell us that a₁₅=420 and then ask what a₁₅ is?