Open question: How does one writing-off and sales book value compute? (Bookkeeping)?

d. 1. July, bought a patent $14,000, so that a singular baking procedure produces a new product. The patent is valid for 15 further years; however the company expects to produce the product for only four years and to marked out. The value of the patent at the end of the four years is zero. What is (writing-off 2008/Tilgung) and (end/time of sale book value) for the patent? Note: The problem did not indicate, which „the method “to use to how I would compute this?