Subtract the second of the following expression from the first
a-b+c-d, c-a+d-b

First: a-b+c-d
Second: c-a+d-b
First-Second
=a-b+c-d-(c-a+d-b)
=a-b+c-d-c+a-d+b  after distributing the negative sign
=a+a-b+b+c-c-d-d  collect similar terms
=2a-0+0-2d
=2a-2d