A sentence having one main clause and one or more sub-ordinate clauses is known as complex sentence.
If both the main clause and sub-ordinate clause have objects, its voice can be changed, as done in assertive sentences. And also understand that a complex sentence may have opposite voices in their clauses.
If the main clause lacks its object, then the voice of the complex sentence is changed in a different way . That is discussed in the explanations of 6th, 7th and 8th questions of the following exercise.
Change the voice.