Hi!
To put something in expanded form, start with the first digit, and turn the rest of the numbers to 0s, add it to the second digit, also turning the numbers to the right to 0s, and si on and so on.
300,000 + 40,000 + 5,000 (the rest are just zeroes so..)
The answer is 300,000 + 40,000 + 5,000
Hopefully this makes sense, and good luck! :)