Answer
General equation is: $a_n=24+(-12)(n-1)$ and
$a_{20}=-204$
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 $a_1=24$
Then, our General equation is: $a_n=24+(-12)(n-1)$
$a_{20}=24+(-12)(20-1)=24-228=-204$
