Hi, I have a problem with below exercise:

Express the solution to each of the following counting problems in terms of a difference equation, then use a generating function to solve the difference equation to give an explicit solution.

I am making a border for the front of a garden bed out of small bricks that are 10 cm long and larger bricks that are 20 cm long. If the small bricks come in two colours, and the larger bricks come in three possible colours, how many different borders of length 10n cm can I make?

Thanks in advance