Answer:
The answer to your question is Ba(OH)₂ + 2HClO₄ ⇒ Ba(ClO₄)₂ + 2H₂O
Step-by-step explanation:
Reactants
Barium hydroxide
Perchloric acid
Formula
Barium hydroxide = Ba(OH)₂
Perchloric acid = HClO₄
Equation (it is a double replacement)
 Ba(OH)₂ + HClO₄ ⇒ Ba(ClO₄)₂ + H₂O
 Reactants Elements Products
 1 Ba 1
 1 Cl 2
 3 H 2
 6 O 9
This reaction is unbalanced
2) Balance the chemical reaction
  Ba(OH)₂ + 2HClO₄ ⇒ Ba(ClO₄)₂ + 2H₂O
 Reactants Elements Products
 1 Ba 1
 2 Cl 2
 4 H 4
 10 O 10
Now the reaction is balanced
Products 
 Ba(ClO₄)₂ Barium perchloride
 2H₂O water