Answer
$\displaystyle \sum_{k=0}^{n}(a+kd)$
(sample answer)
Work Step by Step
(first term) : a+(0)
(2nd term): a+1(d)
(3rd term): a+2(d)
...
If we begin counting (indexing) terms with k=0,
the pattern suggests that the k-th term is built by
adding k(d) to a$:$
$a_{k}= a+kd$
The last term is such that we added $n(d)$ to a, meaning that k=n.
We began with k=0, and ended in k=n:
$\displaystyle \sum_{k=0}^{n}(a+kd)$
(sample answer)