You take 907 and place it above the 728
907
-728
Always start with the ones side. (Right)
You cannot subtract 8 from 7, so you'll need to borrow from a number. Can you borrow from 0? No, because 0 equals to nothing. Can you borrow from 9?
Yes.
So, imagine taking 1 out of 9, and that leaves you with 8 because, well, 9 - 1 = 8.
Now the 0 has a 1, which makes it 10
Which means 10 is greater than 7, so you can borrow a 1 from there.
10 - 1 = 9 So you are left with 9 in the tens column.
10 + 7 = 17, which is greater than 8 so you can subtract that now.
17 - 8 = 9 (Ones column)
10 - 2 = 8 (Tens column)
8 - 7 = 1 (Hundreds column)
So, your answer is 189. Try it on a paper, or simply use a calculator.
Hope this helps! c: