P(B)=P(B1^c\cap B2)=P((B1\cup B2^c)^c)=1-P(B1\cup B2^c)= 1-(P(B)+P(B1^c)-P(B1\cap B2^c))= 1-(\frac{3^n}{6^n} +\frac{5^n}{6^n} - \frac{1}{2^n}) = 1-\frac{5^n}{6^n}