For this problem, we know d = 6 because (11 - 5) = (17 - 11) = (23 - 17) = 6
Now, we have to use the formula:

For this problem, d = 6 and a(base1) is 5, so plug it in like this:

To simplify, all you have to do is distribute the six and reduce like so:
So, here's your answer:
Hope this helps!