There are m simple candidate keys
So every candidate have 2 chance : they can Present in super key OR Absent , But at least one of in 'm' should present .
No. of chance to appear m candidate key in Superset is = (2^m)-1
and all remaining attribute 'n-m' have to chance they can present Or absent in Super key.
No .of chance all remaining attributes to appear in Super Key =2^(n-m)
So , Total no of Super Key Possible=((2^m)-1)(2^(n-m))
( 2^6 -1)( 2^(15-6)) = 32256