The two numbers are very near 7200 and 300. The sum of those is a good estimate of your sum:
... 7200 + 300 = 7500
_____
If you're taught methods of addition that include "make a ten" or "make a hundred", you know you can "borrow" a 1 from 7203 (making it 7202) and loan that 1 to 299 to make it 300. Now, you know your sum is exactly 7202+300 = 7502, with no estimation necessary.