Mark 9:28

And when he was come into the house, his disciples asked him privately, Why could not we cast him out?
— Mark 9:28, King James Version

In other translations

World English Bible (WEB)

When he had come into the house, his disciples asked him privately, "Why couldn't we cast it out?"

American Standard Version (ASV)

And when he was come into the house, his disciples asked him privately, How is it that we could not cast it out?

Young's Literal Translation (YLT)

And he having come into the house, his disciples were questioning him by himself--'Why were we not able to cast it forth?'