First things we are going to add essential prime implicants in every min SOP + Extra min terms we have to add
Here extra min term which is required is 2 .So we have to cover those 2 min terms using extra terms .In order to cover each min terms they are saying that we have 3 choices because they are saying that 2 minterms are in turn covered by 3 non-essential prime implicants each.
So
essential prime implicants +____+____ Here for each Blank we have 3 choices so 3*3 =9
9 Min Sop are possible.