Answer
$ \dfrac{n}{2}(9+5n)$
Work Step by Step
The sum of the first $n$ terms of an arithmetic sequence is given by:
$S_{n}= \dfrac{n}{2}\left(a_{1}+a_{n}\right)$
The terms of the sum are the first $n$ terms of an arithmetic sequence, starting with $a_{1}=7$. The constant difference between the terms is $d=5$
Now, $S_{n}= \dfrac{n}{2}\left(a_{1}+a_{n}\right) \\=\dfrac{n}{2}[7+(2+5n)]$
Therefore, the sum of the first $n$ terms of the arithmetic sequence is: $ \dfrac{n}{2}(9+5n)$
