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