Answer is: Step 1: XY₂+ Z₂ → XY₂Z + Z (slow). Step 2: XY₂+ Z → XY₂Z (fast). Overall reaction: step 1 + step 2: XY₂+ Z₂ + XY₂+ Z ⇌ XY₂Z + Z + XY₂Z. 2XY₂+Z₂ ⇌ 2XY₂Z. The sum of the two elementary steps is equivalent to the stoichiometric equation. Rate of chemical reaction depends on slow step (step 1).