Question

your behaviour surprises me
change into passive voice (omit by)

Answer 1

I am surprised with your behavior ...

Answer 2

The question is answered below-

1. The question requires changing the voice from the active voice to the passive voice along with a condition mentioned.

2. When changed into the passive voice from the active voice, the sentence becomes-

I am surprised at your behavior.

3. As the question said to omit by, at is used to replace by.

4. At and by can be closely used.