If an element has the atomic mass (a) of 80 and its atomic number (z) is 35 how many neutrons

The atomic mass of an atom is primarily comprised of the number of protons plus the number of neutrons.

The atomic number of an atom tells the number of protons present in the nucleus, and is an chemical identifier of the atom.

For z=35, element Bromine, the number of neutrons is equal to the mass minus the number of protons.

80-35 = 45 neutrons in the nucleus.