Question

How to borrow if you can't subtract from the lowest number on top

Answer 1

go to the number next to the one you cannot subtract from, take one away, and add a one in front of the number you cannot subtract from. e.g.
27 1(1)7 the answer would be 9
-8 -8


for your problem the answer would be 1109

Answer 2

8 1
7898
- 6789
=====
1109 is your answer
And to borrow you just subtract from the number before it and add 10