In order to answer that question you need additional informations.
If we assume there is no synergies from the merger (need to make sure of that) you'll need to have revenues of both companies or at least the ratio of revenues between the two companies.
So if company 1 has a revenue of A $ and company 2 has a revenue of B$ then the new margin will be in percent: margin of the company 1( 6.4*A)+ margin of the second company (3.8*B) and divide the total by the total revenues of the new entity (A+B).
If you only have the ratio between the two revenues then that can works as well: if you know that company 1 has X times the revenue of company 2 then, the new margin will be (in percent):
margin of company 1 with respect to company 2 (which is equal to X*6.4)+ margin of company 2 with respect to company 2 (which is equal to 3.8) and you devide this sum by the total revenues of the company with respect to company 2 (which is X+1).