#### Answer

$3$

#### Work Step by Step

From the graph we get
$a_{1}=-4;a_{2}=-1;a_{3}=2;a_{4}=5;a_{5}=8$
So the common difference is $d=a_{n+1}-a_{n}=a_{5}-a_{4}=a_{4}-a_{3}=a_{3}-a_{2}=a_{2}-a_{1}=8-5=5-2=2-\left( -1\right) =\left( -1\right) -\left( -4\right) =3$