marking scheme of question 1. 
For 1a), adjunct of B (2 points)
         adj(B)= [  -24 12 -6 
                    20 -10 -4
                    -15 -6 3];
 
         det(B)=-54      (1 point)
         the inverse of B (1 point)
       
        common mistake:
           the sign of the entries of the adjunct of B.
       
For 1b), state the rule ( 1 point)
         det(A)=-54  (1 point- though as above)
         det(Bi)=-6 (1 point)
         y=1/9 (1 point)
        common mistake:
           Forget the statement of rule;
           some calculation mistakes.