"Most people also think that pirates have been exisiting for hundreds of years"
Sounds good to me, but I'm not a native English speaker.
Pirates existed hundreds of years ago and they're still existing, so my guess would be the same.
Let's see what the experts say about this.
And, since you asked to correct ANY mistakes:
"I'm not sure whether I did the exercise correctly or not."
"Could you help me?" (you're asking for help politely)
Student or Learner