Question

3) People built a bridge over the river. 4) Peter finished the composition. 5) A lot of people planted these flowers. ПРЕВРАТИТЕ ЭТО В PASIVE

Answer 1

A bridge (over the river) was built by the people (не уверена насчёт the)
The composition was finished by Peter
These flowers were being planted by a lot of people

Answer 2

3) A bridge was built over the river by people. 4) The composition was finished by Peter. 5) These flowers were planted by a lot of people.