[bmu123]

Struggling with this past paper question. So I know it's first order from the rate constant units. And as k_obs has a wide range it happens by the associative mechanism. Or interchange where bond formation dominates?
I have in my notes that the k_obs for an associative mechanism is:
k₁[NH₃]/k_-1/(k₂ + 1) and I'm really confused as to how I'm supposed to end up with k₁ + k₂ [NH₃] and how I work out what k₁ and k₂ are from there. Any help would be much appreciated.