Br-73
The 73 means that the number of protons + neutrons = 73
We can find the number of protons by looking at a periodic table as that is unique to each element. Bromine has 35 protons.
It will have 73 - 35 = 38 neutrons.
Since this is a neutral element, the number of electrons will be equal to the number of protons. It will have 35 electrons.
35 protons, 38 neutrons, 35 electrons.