Help with long subtraction of multiple lines of numbers

Been a long time since I did any subtraction manually and I've forgotten most of the rules. I understand the basic concept of borrowing and carrying over when doing two lines of numbers but what about three or four lines? Specifically.. what about cases where the numbers below the number you have added onto still add up to more than the number on top after borrowing... For example:

- 7815
3784
1642

For the first line on the right I'd borrow from the 1 next to it to make it 15 minus 6 which is 9 but on the second right line when borrowing the number becomes ten and the bottom two numbers add upto more than than ten.. what do I do in this instance? Do I need to break up the equation in to two separate equations or is there a way to do the whole thing?

You could break the up the equation into two separate equations. You could also do the whole thing:
07815
03784
-1642
You borrow 1 from 1 (in 7815) and 5 becomes 15.
15 - 4 - 2 = 9
You borrow 2 from 8 ( in 7815), the number becomes 20.
20 - 8 - 4 =8
In the second column 8 (in 7815) becomes 6 (since we'd borrowed 2 from it).
You borrow 2 from 7 (in 7815). The number becomes 26.
26 - 7 - 6 = 13
You write 3, adding the 1 to 5 (we borrowed 2 from 7) to become 6
6 - 3 - 1 = 2
Answer: 2389

i have never subtracted 3 number at the same time or less i could figure it out in my head or use a calculate but for that problem i think you need to break it up

Well, Dogbox, the Order of Operations states that Multiplication, Division, Addition then Subtraction be carried out in that order. So, complete all the Addition operations then follow with the Subtraction.

If I understand your entry correctly it is: -7815 + 3784 + 1642. Correct? If so, simplify it by first doing the addition:
3784
+1642
Begin by adding the last digits in the columns, 4 + 2 = 6. Write the 6 under the far right hand column.