If an element has atomic number 9 and atomic mass 19, how many neutrons will it have?
To an excellent approximation, atomic mass is equivalent to nuclear mass; i.e. the combined mass of neutrons and protons. Can you tell us the identity of the nuclide? You have all the info necessary to do so.
We know that Mass number
Mass Number = Number of Protons + Number of Neutrons;
And Atomic Number = Number of Protons.
So Number of Neutrons = Atomic Mass - Atomic Number = 19 - 9 = 10
This explains the answer.