Answer
The charge on the inner surface of the shell is $~~+4~\mu C$
Work Step by Step
The charge on the outer surface is $-14~\mu C$
The net charge on the shell is $-10~\mu C$
We can find $q$, the charge on the inner surface of the shell:
$q+(-14~\mu C) = -10~\mu C$
$q = -10~\mu C+14~\mu C$
$q = +4~\mu C$
The charge on the inner surface of the shell is $~~+4~\mu C$
