Question: arithmetic sequence formula from a seq

I want to find the formula for seq: 

För example 



How do i revers i backword fo get arithmetic sequence formula. I now the answare but want to learn how it's done in maple. I tried the app but i was not able to solve it. I have read the help and it looks like you have to use rsolve. Does anyboy has a sample to do not is trail and error. 

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