Answer
a. $Mg + Br_{2} → MgBr_{2}$
b. $Ca + Br_{2} → CaBr_{2}$
c. $Sr + Br_{2} → SrBr_{2}$
Work Step by Step
Mg, Ca & Sr each react with Br to form corresponding compound. Note, the number of atoms of each element, along with net charge, must be conserved.