The notation [tex]^{19}_9X[/tex] refers to some element X that has 9 protons and 19 protons and neutrons (so that means 19-9 = 10 neutrons). The element that has 9 protons is fluorine (symbol F on the periodic table)
This means [tex]^{19}_9X[/tex] would update to [tex]^{19}_9F[/tex]
