You have to borrow and regroup in your problem. You can't do 4 minus 7, so you borrow 1 from the number to the left, which is 0, so we go to the next number on the left, 6.
6 becomes 5 and the 4 becomes 14. The 0 becomes a 9, too. Now you can subtract. 14 minus 7 is, well, 7. 9 minus 5 is 4. 5 minus 3 is 2.... and you get your answer: 247.
Hope this helped you.. but if you need help, let me know!