Balancing a chemical equation is like a recipe where you must make sure every ingredient used as a reactant matches the exact count in the final dishes served.

Count atoms on left and right, balance each element with numbers right!

This is a double displacement reaction (also known as a precipitation reaction). Aluminium sulphate reacts with barium chloride to form aluminium chloride and a white precipitate of barium sulphate. To balance it: start with 3 sulphate groups on the left, requiring 3 $\text{BaSO}_4$ on the right, which in turn requires 3 $\text{BaCl}_2$ on the left, and finally 2 $\text{Al}$ atoms on both sides yielding 2 $\text{AlCl}_3$.

When balancing equations containing polyatomic ions like $\text{SO}_4^{2-}$, treat the entire ion as a single unit if it does not undergo change during the reaction.