You need to know the mass number to work it out. I can see you have it but it's not visible on your photo.
Every proton weighs 1 and every neutron weighs 1 so if you know the total mass of the nucleus and the number of protons, then you can do:
Mass Number - Proton Number = Neutron Number
because the rest of the mass (that isn't from the protons) must come from the neutrons. Make sense?
