Answer
See the answer below.
Work Step by Step
The rate of the slow step is the determining rate:
$r=r_S=k_S[H_2][I]^2$
For the fast equilibrium:
$r_{-F}=k_{-F}[I]^2=r_F=k_{F}[I_2]\rightarrow [I]^2=k_F/k{-F}[I_2]=K_F[I_2]$
Overall rate:
$r=(k_SK_F)[I_2][H_2]$
