Another answer:

15 - 9 = 6

    +---------        |       |        |                 +---------+
    |                 1       |        |                 |         |
    |                 |       |        |   -----------   |         3
    +--------+   -----+-----  +--------+                 +---------+
             |        |                |   -----------   |         |
             |        2                |                 |         |
    ---------+        |                |                 +---------+

becomes

    +  +---------                +---3----+                 +---------+
    1  |                         |        |                 |         
    |  |                         |        |   -----------   |         
    +  +--------+   -----+-----  +--------+                 +---------+
    |           |                         |   -----------   |         |
    2           |                         |                 |         |
    +  ---------+                         |                 +---------+