The solution of this question is given by assuming step (i) to be reversible which is not given in question
Overallrate=Rateofsloweststep(ii)
=k[X][Y2] ......(i)
K=rate constant of step (ii)
Assuming step (i) to be reversible, its equilibrium constant,
keq=[X2][X]2⇒[X]=keq21[X2]21 .....(ii)
Put (ii) in (i)
Rate =kkeq21[X2]21[Y2]
Overall order =21+1=23