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 be viewed only if
  1. The questioner was satisfied and accepted the answer, or
  2. The answer was disputed, but the judge evaluated it as 100% correct.
View the answer
The answer is accepted.