Hi! I'm happy to help!
To solve this, we first need to know what scientific notation is. Scientific notation is a way a number is written. You write it using a number ranging from 1 to <10. Then, you multiply this number by [tex]10^{x}[/tex].
In order to make this number reasonable for scientific notation, you have to move the decimal a certain amount of places. In this case you move it 3 places to the right to make your number 2.349. Since we moved it 3 places to the right our x will be -3. (If we'd moved it 3 places to the left, our x would be positive 3.)
So now, we insert our numbers:
2.349·[tex]10^{-3}[/tex]
You write 0.002349 like 2.349·[tex]10^{-3}[/tex], when it is in scientific notation.
I hope this was helpful, keep learning! :D