This looks liks an online quiz, and therefore to be done for a grade/marks. It is forum policy not knowingly to help with such questions.
I need help in doing D and E since I am completely stuck and do not know how to proceed. For D though, I suspected at first that it would involve a derivative but then I read the "The formula should not involve A or x ." part which shut down that idea. As for E, it depends on D so I can't understand it at all!
If you have a specific question to ask me to get to know my perspective on the problem, just ask.
Any help would be greatly appreciated!
Thanks in advance!