Write the reaction in two half reactions,oxidation half and reduction half as:
Oxidation:
Zn→Zn(OH)−24Reduction:BrO−4→Br
Now balance the oxygen atoms by adding H2O molecules.
Zn+4H2O→Zn(OH)−24
BrO−4→Br−+4H2O
Now balance hydrogen atoms by adding H+ ions.
Zn+4H2O→Zn(OH)−24+4H+BrO−4+8H+→Br−+4H2O
As reaction takes place in basic medium, add one OH− each side for every H+ ion present in the reaction.Combine OH− and H+ to form H2O.
Zn+4OH−→Zn(OH)−24BrO−4+4H2O→Br−+8OH−
To balance the charge,add electrons to more positive side to equal the less positive side of the half reaction.
Zn+4OH−→Zn(OH)−24+2e−BrO−4+4H2O+8e−→Br−+8OH−
Now multiply oxidation half reaction by 4 and add both the reactions we will get
4Zn+BrO−4+4H2O+8OH−→4Zn(OH)−24+Br−