Class B has 16 network bits and rest 16 host bits.
Subnettting is done by chossing bits from host. So in order to make 64 subnets we have to carry log2(64)= 6bits from host.
Now look NID.NID.XXXXXXXX.XXXXXXXX we carry 6 bits and we know subnet mask contain 1 for NID and subnet bits and 0 for host bits so Mask will be 11111111.11111111.11111100.00000000 : 255.255.252.0