The question ask to find a formula for the sum of the first n terms of the sequence

9,13,17,21,....

So the answer is Sn= n(2n + 7)

Can someone explain why would this arithmetic sequence of sum not look like the usual formula, that is Sn= [n(a1 + an)] / 2

I think Im missing a key point in understanding whats going on here.

Thanks in advance