find cardinality of superset from its subsets with |6|

Jan 2017
11
0
USA
did I solve this correctly?

problem:
|{X: X∈P(B), |X| = 6}| = 28, find |B|

solution:
C(n, 6) = n! / 6!(n - 6)! = 28
C(8, 6) = 28
|B| = 8