P(B)=P(B1^c\cap B2)=P(B1^c\cap(B2^c)^c)=P(B1^c\cap (B2^c\cap B1^c)^c)=P(B1^c\backslash (B1^c\cap B2^c))=P(B1^c)-P(B1^c\cap B2^c)=\frac{1}{2^n}- \frac{1}{3^n}