The 4th and 13th terms of an ap are 5 and -1, find the 8th term of an ap

Now, we know the 4th term is 5.... ok... now, what's the common difference?  well, we dunno, but notice, from the 4th to the 13th term, you do have to use it 9 times to hop over to the 13th term, let's say is "d", then



we also know that the 13th term is -1



now, recall above what the 8th term is 

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