Chapter 11 - Further Topics in Algebra - 11.2 Arithmetic Sequences and Series - 11.2 Exercises - Page 1024: 7

$3$

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 $d=a_{n+1}-a_{n}=a_{5}-a_{4}=a_{4}-a_{3}=a_{3}-a_{2}=a_{2}-a_{1}=8-5=5-2=2-\left( -1\right) =\left( -1\right) -\left( -4\right) =3$