Answer
$v_{ac}=16V$
$v_{cd}=-24V$
Work Step by Step
Using the diagram, we can apply KVL to the loop areas:
$v_{ab}-v_{cb}-v_{ac}=0, v_{ac}=12-(-4) V=16V$
$v_{ab}-v_{cb}+v_{cd} +v_{da}=0, 12-(-4)+v_{cd}+8=0, v_{cd}=-24V$
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