Work with me here, to start these problems, you need an expression for the equilibrium constant, or K
eq as the question mentions, note that for a general reaction
aA + tT
cC + dD
where a, t, c and d are the stoichiometric coefficients of the reactants, and A, T, C and D are the chemical species involved. For example, the stoichiometric coefficient for I
- in your equation is 3.
For this general equation, the equilibrium expression is K
eq = [C]
c[D]
d / [A]
a[T]
t where the [] indicates the concentration
Now if you were to apply this to your question, what would be the expression for K
eq?