Respuesta :

a(n)=a(1)+d(n-1)

a(1)=8

a(4)= 8+d(4-1)
17=8+d*3
17-8=3d
9 =3d
d=3

a(n)= 8 +3(n-1)   Sometimes it is required like this.

Sometimes , it is required to simplify more
a(n)=8+3n-3=5+3n
a(n)=5+3n