New Living Translation
Afterward, when Jesus was alone in the house with his disciples, they asked him, “Why couldn’t we cast out that evil spirit?”

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

Berean Study Bible
After Jesus had gone into the house, His disciples asked Him privately, “Why couldn’t we drive it out?”

Douay-Rheims Bible
And when he was come into the house, his disciples secretly asked him: Why could not we cast him out?

English Revised Version
And when he was come into the house, his disciples asked him privately, saying, We could not cast it out.

King James Bible
And when he was come into the house, his disciples asked him privately, Why could not we cast him out?

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

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

Mark 9:28 Additional Translations ...