Answer
General equation is: $a_n=-20+(-4)(n-1)$; and
$-96$
Work Step by Step
Formula to calculate the nth term of an arithmetic sequence is: $a_n=a_1+d(n-1)$
Here, $a_1: $ First term and $d=$ common difference
Since, we have $a_1=-20$
Then, our General equation is: $a_n=-20+(-4)(n-1)$
$a_{20}=-20+(-4)(20-1)=-20-76=-96$
