Answer
$n(2n-3)$
Work Step by Step
$7-3=4$
$3-(-1)=4$
There is a constant difference, $4$, between terms.
The terms of the sum
are the first n terms of an arithmetic sequence, $a_{1}=-1, d=4.$
Sum of the First $n$ Terms of an arithmetic sequence:
$S_{n}=\displaystyle \frac{n}{2}\left(a_{1}+a_{n}\right)$
$=\displaystyle \frac{n}{2}(-1+(4n-5))$
$=\displaystyle \frac{n}{2}(4n-6)$
$=\displaystyle \frac{n}{2}\cdot 2(2n-3)$
$=n(2n-3)$
