Arithmetic Sequences Help

I've been struggling with these two questions  for the entire day and I just can't seem to understand how to solve it, please provide clear instruction on how you solved it basically explain as much as you can.


Answers can only be viewed under the following conditions:
  1. The questioner was satisfied with and accepted the answer, or
  2. The answer was evaluated as being 100% correct by the judge.
View the answer
The answer is accepted.
Join Matchmaticians Affiliate Marketing Program to earn up to a 50% commission on every question that your affiliated users ask or answer.