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
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.