This is an arithmetic sequence because the difference
between the terms in the sequence remain constant.
In other words, we subtract 9 from one term to get the next.
So we start off with the explicit formula, shown below in yellow.
"n" will be the number of terms in the sequence,
a1 will be the first term in the sequence,
and d will be the common difference.
Now substitute these in like I have below.
You will get -425 as an answer.