Question

who has taught you to play the piano? voice change.

Answer 1

By whom have you taught to play the piano?

Answer 2

Question :

Who has taught you to play the piano ?

( Change the voice )

Answer :

By whom have you been taught to play the piano ?

Rules :

Here, 'who' changes into 'By whom' in passive voice.

The sentence is in present perfect tense. Here, 'has' changes into 'have been' and the sentence being interrogative one, it takes a new form as seen in the answer.

#_100% correct✌