Chapter 9 - Section 9.2 - Arithmetic Sequences - 9.2 Assess Your Understanding - Page 654: 41

$\displaystyle \frac{n(9+5n)}{2}$

There is a constant difference, $5$, between terms. The terms of the sum are the first n terms of an arithmetic sequence, $a_{1}=7, d=5.$ Sum of the First $n$ Terms of an arithmetic sequence: $S_{n}=\displaystyle \frac{n}{2}\left(a_{1}+a_{n}\right)$ $=\displaystyle \frac{n}{2}(7+(2+5n))$ $=\displaystyle \frac{n}{2}(9+5n)$