The sum of the first n terms of a sequence is Sn = 3n² + 5n…

Step 1. The first term. $a_1 = S_1 = 3 + 5 = 8$.

Step 2. The general term. $S_{n-1} = 3n^2 - n - 2$, so $a_n = S_n - S_{n-1} = 6n + 2$. It gives $a_1 = 8$ too.

Step 3. The tenth term. $a_{10} = 62$.

Step 4. An AP? $a_n$ is linear in $n$, so yes, with common difference $6$.