Fe and S are already balanced in this equation
Further explanation
Balanced indicates that the number of elements in question has the same number of atoms both on the left side (the number on the reactants side) and on the right side (the number on the product side)
Even though one of the elements is said to be balanced, the chemical equation as a whole cannot be said to be balanced, because the chemical equation has several components of the constituent elements.
To balance chemical equations, a coefficient in front of the compound is usually added either in the reactants or products
Reaction
FeS₂ + Cl₂ → FeCl₃ + S₂Cl₂
From this equation, it can be shown that: the Fe and S elements are balanced because the numbers are equal both on the left and right, while the Cl elements are not yet balanced
Fe, left=1, right=1 ⇒ balanced
S, left=2, right=1 ⇒ balanced
Cl, left=2, right=5⇒ not balanced
For balanced equations:
2FeS₂ + 5Cl₂ → 2FeCl₃ + 2S₂Cl₂