Answer:

Did you plant these flowers ?

Were these flowers planted by you ?

1▪ Subject of active voice changes to object of passive voice

2▪Object of active voice changes to subject ofcpassive voice

3▪Use of third form of verb

4▪Use of "by"

5▪ helping verb is borrowed from upcoming tense

As in above sentence helping verb is in past simple so in passive voice helping verb will be from Past continuous .