Prove by Mathematical Induction

Hi everyone I need a little help, Im stuck.

I have to prove by induction that:

is exactly divisible by 7 for all positive integers of n.

Now what I have done is:

1)

If is divisible by 7

=>( )-----> Pn statement

where a is a positive integer

2) statement where n=1

=>

LHS:

RHS:

Therefore is true.

3) Assuming to be true where n=k

=>

4) Therefore

Now here is where Im stuck, Im not getting to factor out a 7 i.e.

getting 7(some integer) to prove that is divisible by 7.

Any help would be greately appreciated.