Answers :
 a₀ = 3
 a₁ = 1
 a₂ = 1
 a₃ = 3
Explanation :
 
To balance the reaction equation, both left and right hand sides should have same number of atoms in each element.
Given reaction is
a₀HClO₄ + a₁Pb(NO₃)₃ → a₂Pb(ClO₄)₃ + a₃HNO₃
Here, 
 Left Hand Side has  Right Hand Side has
 H = 1 atom H = 1 atom
 Cl = 1 atom Cl = 3 atoms
 O = 13 atoms O = 15 atom
 N = 3 atoms N = 1 atom
 Pb = 1 atom  Pb = 1 atom
Pb is already balanced. Then others should be balanced by adding coefficients.
First step : balance the N atoms in both sides. 
There are 3 N atoms in left hand side and 1 N atom in right hand side. Hence, to balance N atoms "3" should be added before HNO₃. 
 
Second step : balance the Cl atoms.
There are 3 Cl atoms in right hand side while 1 atom in left hand side. So, "3" should be added before HClO₄ to balance Cl atoms in both sides.
After doing those, number of O atoms and H atoms in both sides is automatically balanced.