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