“I'm on it.”
You're in a meeting at work. Someone complains that there's a problem with the software system that they use. You're one of the people who's repsonsible for this software, so you let everyone know that you will fix this problem.
I'm on it.
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"I'm on it" means "I will deal with this" or "I will handle this problem." You can use it in work situations:
A: Bobby, there's something wrong with the phone system. Can you look into that?
B: I'm on it!
"I'm on it" sounds helpful and positive. When you say this, it seems like you're in control and you've already started to fix the problem.