Sorry, the textbook is not clear. Let's collect all these bugs to make our lives easier.
Most people are doing well.
One problem:
"For a causal system (A,E,D), A = A*E ..."
There is no assumption here whether the system is causal or not. And you don't really need this condition at all. The problem statment says "input process A has an envelope of E". You can only have A < = A*E.