Answer
$-7920$
Work Step by Step
There are $90$ terms, and they form an arithmetic sequence with
$d=a_{n+1}-a_{n}=3-2(n+1)-(3-2n)$
$=3-2n-2-3+2n$
$=-2$
$a_{1}=3-2(1)=1,$
$a_{90}=3-2(90)=-177$
Sum of the First $n$ Terms of an arithmetic sequence:
$S_{n}=\displaystyle \frac{n}{2}\left(a_{1}+a_{n}\right)$
$S_{90}=\displaystyle \frac{90}{2}(1+(-177))=45(-176)=-7920$