Hehe, I really went to the website to search their standard answers. It turned out that there were really mistakes, and the people who asked and answered these questions were really careless.

My calculation is this:

D(2b-2， 1)E(3，b-3/2)，

∴S=S = s moment -(s △ OCD+s △ OAE+s △ DBE)

=3- 1*(2b-2)/2-(5-2b)(5/2-b)/2- 1*(b-3/2)/2=b^2-3b/2-3/2