Answer
$a_{n}=20-3n$
$a_{20}=-40$
Work Step by Step
The sequence is arithmetic, because there is a common difference between terms,
$d=-3,\quad a_{1}=17.$
Write the general rule
$ a_{n}=a_{1}+(n-1)d\qquad$ ... substitite $a_{1}$ and $d$
$ a_{n}=17+(n-1)(-3)\qquad$ ... simplify
$a_{n}=17-3n+3$
$a_{n}=20-3n$
For n=20,
$a_{20}=20-3(20)=-40$
