The sequence a1 = 6, an = 3an-1 can also be written as

The question as presented is incomplete, here is the complete question with the multiple choice:

The sequence a1 = 6, an = 3an − 1 can also be written as:

1) an = 6 ⋅ 3^n
2) an = 6 ⋅ 3^(n + 1) 
3) an = 2 ⋅ 3^n
4) an = 2 ⋅ 3^(n + 1)

The correct choice is option 3) an = 2⋅3^n.

If we look at the initial sequence an = 3⋅an-1, and

a1 = 3⋅a0 = 6
a0 = 6/3
a0 = 2

We can now look at the sequence.

a0 = 2
a1 = 6
a2 = 18
a3 = 54
etc...

A common factor in each of those numbers is 2, so we can rewrite the sequence by factoring out 2.

a0 = 2⋅1
a1 = 2⋅3
a2 = 2⋅9
a3 = 2⋅27

The numbers being multiplied by 2 are all factors of 3. So we can rewrite the sequence again as:

a0 = 2⋅3^0
a1 = 2⋅3^1
a2 = 2⋅3^2
a3 = 2⋅3^3

This sequence can now be rewritten as an = 2⋅3^n.