#### Answer

$d=-4$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To find the common difference of the given arithmetic sequence, $
-8,-12,-16,-20,...
,$ subtract a term by its previous term.
$\bf{\text{Solution Details:}}$
Since the common difference of an arithmetic sequence is the difference between a term and its previous term, then with $a_2=
-12
$ and $a_1=
-8
$, the common difference, $d$, is
\begin{array}{l}\require{cancel}
d=-12-(-8)
\\\\
d=-12+8
\\\\
d=-4
.\end{array}