What radionuclide decays to Br-73 by positron emission?

Answer 1

Work backwards. Positron emission means (essentially) a proton decayed into a neutron/positron pair. The mass number remains the same, but the atomic number goes down one to Bromine.

Krypton has an isotope that fits this bill.

Answer 2

Krypton-73 decays to Bromine-73 by positron emission. In this process, a proton is converted into a neutron, releasing a positron and a neutrino.