you need to subtract the given addend from the sum:-
8d5 - 3c3d2 + 5c2d3 - 4cd4 + 9 - (2d5 - c3d2 + 8cd4 + 1)
Now we remove the parentheses - Note that the signs of the terms in the parentheses change because of the negative in front of the paranthese):_
8d5 - 3c3d2 + 5c2d3 - 4cd4 + 9 - 2d5 + c3d2 - 8cd4 - 1
Now adding like term we get
6d5 - 2c3d2 + 5c2d3 - 12cd4 + 8
This is the other addend