a
 5 - ---
      b
---------
   15
  ----
    b

Put the 5 over 1

  5     a
 --- - ---
  1     b
-----------
    15
   ----
     b

The LCD of all 3 denominators is b.
So multiply top and bottom by b/1
     _         _ 
 b  |  5     a  |
---·| --- - --- |
 1  |_ 1     b  |
-----------------
 b  |¯   15    ¯|
---·|   ----    |
 1  |_    b    _|


Distribute out the top and
multiply out the bottom:

  5b - a
  ------
    15


Edwin