These isotopes of Bromine and Strontium have the same mass number. Therefore the radioactive process from Br to Sr must be beta decay. It cannot be alpha decay, since alphas reduce the mass number by 4.

The atomic numbers of Br and Sr are 35 and 38 respectively, as you can see from the Element Tables. Each beta decay increases the atomic number by 1, so there must be 3 beta decays to get from 35 to 38.

The beta decay of ⁹⁰₃₅Br will lead to a nucleus with atomic number 36. From the Tables you see that this nucleus is Krypton (symbol Kr). Beta decay does not change the mass number, so this will be 90. The decay must be

Next there is a beta decay of Kr, leading to the nucleus with atomic number 37 and mass number 90. This nucleus is Rubidium (Rb). The decay is

Finally, Rb decays, leading to Strontium-90:

The first two decays are fast (the half-lives are short). But ⁹⁰Sr has a long half-life (30 years) and so there is a kind of bottleneck; the ⁹⁰Sr accumulates.