Answer
$-41J$
Work Step by Step
As we know that
$\Delta U_{AB}+\Delta U_{BC}+\Delta U_{CD}+\Delta U_{DA}=0$
This can be rearranged as:
$\Delta U_{CD}=-\Delta U_{AB}-\Delta U_{BC}-\Delta U_{DA}$
We plug in the known values to obtain:
$\Delta U_{CD}=-82-15-(-56)=-41J$
