Answer
All the charge $q_a$ must be on the outer surface of the smaller shell.
The charge on the inner surface of the larger shell is $-q_a$
The charge on the outer surface of the larger shell is $q_b+q_a$
Work Step by Step
The spherical shells are conductors so there is no electric field within the shells. Therefore, the charge on the inner surface of the smaller shell is zero. All the charge $q_a$ must be on the outer surface of the smaller shell.
Since there is no electric field within the larger shell, the net charge of the smaller shell plus the inner surface of the larger shell must be zero. Therefore, the charge on the inner surface of the larger shell is $-q_a$
Then the charge on the outer surface of the larger shell is $q_b+q_a$. Note that the net charge on the larger shell is then $q_b$
