@Marcia102
Hi - it's important to understand the difference between an enquiry and a request. A request will give you the option to pre-approve or decline, which you must do within 24 hours. So you should decline the guest looking for a shorter stay, with your explanation, and leave the option for the longer term guest to book.
Have you pre-approved the longer term guest? If not then do so - or if it was just an enquiry then no further action is necessary. It's also important to remember that many hosts get overlapping enquiries and requests - how you decide to prioritise these is up to you - many hosts go with first come first served, as guests are usually shopping around and the better booking you are waiting for may never materialise