hi mam,

I am confused on how to do this problem!

∆Grxn 0 = ∑[∆Gf(S) 0 + ∆Gf(O2) 0  ]  - 2∆GSO3 0 =  [2*0 +0] –[2*-370.4]


Now ΔGf0of any element in its stable form is zero.
For S and O₂ it is zero and plug in the value of SO₃ to get the answer.