An attentive reader should notice that the targeted collection of books 
(2 music books and 6 Biology books) can be obtained from the original set of books 
(4 music books and 6 Biology books) by removing 2 music books.


Therefore, we subtract the mass of 2 music books (3.4 kg) from the mass 
of the original set of books (7.2 kg), and we obtain the answer immediately

    7.2 - 3.4 = 3.8 kilograms.


ANSWER.  The total mass of 2 music books and 6 Biology books is 3.8 kilograms.