I 140 Denial work experience

my priority date is oct 2007, Got an RFE requiring ability to pay, must prove 36 months of work experinces. I responded to the RFE but i only got 14 months of experience because my previous employer didnt give me the employment verification, though the total number of months i worked as a manager is 39 months, Im planning to file an appeal and got a notarized affidavit from my previous Chief operating officer proving that i worked in their company for 25 months. My question is, would that affidavit from my previous COO is a good or enough proof to prove and qualify for the 36 months requirement, if not what are the alternative documents I can use for the appeal? and lastly, would you recommend that i get an expert opinion or work experience evaluation? Thank you very much