[tex]a_1=-4\\ a_2=1[/tex]
[tex]\boxed{a_n=a_k+(n-k)r}[/tex]
[tex]a_2=a_1+r\ \therefore\ r=a_2-a_1=1-(-4)\ \therefore\ \boxed{r=5}[/tex]
[tex]a_6=a_1+5r=-4+5(5)\ \therefore\ \boxed{a_8=21}[/tex]
