I presume you meant

Can you use induction?

Let n = k = 1, then

which is, of course, divisible by 7.

(Or if you want to do it "a modulo" then

Thus

Now, and , so

Now you need to show that if for n = k

That for n = k + 1

You can do that by, say, finding from the n = k part of the statement, and subbing that into the n = k + 1 part.

-Dan