Answer
$a_{21}=-121$
Work Step by Step
Using $a_n=a_1+(n-1)d$ or the formula for the $n^{th}$ term of an arithmetic sequence, with $a_1=19$, $d=-7,$ and $n=21,$ then
\begin{align*}
a_{21}&=19+(21-1)(-7)
\\&=
19+(20)(-7)
\\&=
19-140
\\&=
-121
.\end{align*}
Hence, the $21^{st}$ term of $19,12,5,...,$ is $a_{21}=-121$.
