I assume you have to balance the reaction?
a Fe + b H₂SO₄ → c Fe₂(SO₄)₃ + d H₂
Count the number of times an element occurs on either side of the reaction. These numbers must be equal.
Fe: a = 2c
H: 2b = d
S: b = 3c
O: 4b = 12c
Two of these equations depend on c, so we can treat it as a free variable. Let c = 1. Then a = 2 (Fe) and b = 3 (S), from which it follows that d = 6 (H). So the balanced reaction is
2 Fe + 3 H₂SO₄ → 1 Fe₂(SO₄)₃ + 6 H₂
