Answer:
2HBr + Mg(OH)2 => MgBr2 + 2 H2O
Explanation: To balance the equation put 2 in H2O in the product side. Then balance the other Hydrogen atoms by adding 2 in HBr. Since there is another 2 atoms of H in (OH)2 so hydrogen atoms are balanced having 4 atoms each sides. Also Mg and Br are balanced