Another answer:
15 - 9 = 6
From
. +--------- | | | +---------+
. | 1 | | | |
. | | | | ----------- | 3
. +--------+ -----+----- +--------+ +---------+
. | | | ----------- | |
. | 2 | | |
. ---------+ | | +---------+
becomes
. + +--------- +---3----+ +---------+
. 1 | | | |
. | | | | ----------- |
. + +--------+ -----+----- +--------+ +---------+
. | | | ----------- | |
. 2 | | | |
. + ---------+ | +---------+