Find it estimate P (5) P (10) P (7.75) and P (9.8)?

Wrong question. A function is any process that takes a qualified input number and give a single value output number. For instance a square root is not a function because each input gives two output numbers. The list of all qualified input numbers is called domain and the list of all possible output numbers is called range. So "domain" does not qualify for what you have there. You have a range of numbers.