Give the details of finding the missing numbers for subtractions
For 5 _ 6 _
2 7 _ 8
--------------------
_ 6 1 3

Give the details for finding the missing number.
for example  5___6___ is the minuend
                   --  27___8 is the subtrahend 
                       -------------
                       ___613 is the answer   
  
    please give the answer quickly.